The most common cause is anti virus programs scanning outgoing mail and outgoing attachments. Try it with any such scanning disabled.

I should mention that this timeout occurs at exactly 20 seconds from clicking Send.

The time frame is not all that relevant... An extended period (In computer term a second is a long time) of non activity is and anti virus scanning often causes that..

NIS email protection turned off for test. No change. Same error message.

NIS=Norton Internet Security. Any other suggestions?

Well I hope you have anti virus scanning of email disabled in NIS permanently. It rarely actually scans mail these days, being unable to scan mail on SSL or TLS secured connections (most of them) or IMAP accounts, also the most common access method.

when an email is send, it is assembled in the temp folder. my guess is NIS is scanning the email there as a file as well as soon as it is created. Given the decades old habit of NIS slowing everything on a computer to a crawl, I suggest you try with a difference virus solution and see if things get better.

In that NIS email protection was completely disabled for the test, it seems unlikely to me that NIS is the cause of this attachment in excess of 500 KB problem. I suspect that it's the byproduct of a recent Tbird update. Does anyone know which timeout settings in config editor could impact this problem.

Been using NIS on dozens of computers for years without experiencing the performance issues you're describing, Matt. Norton had a top of the line product that was a resource hog, but not NIS. Don't want to see this thread go off topic but what's your choice for Internet security, Matt.

We have 1 person with a timeout issue. You. so what are the odds we have a bug and only 1 of the 25 million or so users is affected? I would be guessing very remote.

That Nortons lies about what is enabled and fails dismally to remove it self from memory even when it is reporting same is a known fact. That you have used NIS exclusively you have no idea how much it slows machines. You would need to have identical devices with different solutions to be even able to guess at the probl;ems one causes over the other.

So I suggest;

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Your recommendation for addressing the timeout error for attachments over 500 KB worked on two of the three computers experiencing the problem. It turned out to be the Adobe Acrobat plugin causing the error message on those machines. Unfortunately, restarting Tbird in safe mode and consequently restarting my computer in safe mode resulted in the same error message. I've attached a screen grab of the error message so that you can see if it's being generated by Tbird. Additional suggestions would be appreciated.

As a reminder, Outlook Mail does not have an issue with large attachments. This problem is unique to Tbird.

I can only assume we have a setting issue then. Given Microsoft clients go for the minimum security and Thunderbird tends to go for the highest. it will probably be in the SSL/TLS or password transmission type.

Please add the Mail and News Accounts from troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Application Basics

   Name: Thunderbird
   Version: 45.5.1
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20161130024037
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account2:
     INCOMING: account2, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account3:
     INCOMING: account3, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account5:
     INCOMING: account5, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account6:
     INCOMING: account6, , (pop3) pop.metro100realty.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) pop.metro100realty.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account12:
     INCOMING: account12, , (pop3) pop3.hughes.net:110, plain, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account21:
     INCOMING: account21, , (none) Local Folders, plain, passwordCleartext

   account22:
     INCOMING: account22, , (pop3) pop.metro100realty.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account24:
     INCOMING: account24, , (imap) imap.googlemail.com:143, plain, passwordCleartext
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true

   account25:
     INCOMING: account25, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true

   account26:
     INCOMING: account26, , (pop3) pop.metro100realty.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account27:
     INCOMING: account27, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account29:
     INCOMING: account29, , (pop3) pop.gmail.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

   account30:
     INCOMING: account30, , (pop3) mail.dynamicmediasolutions.us:995, SSL, passwordCleartext
     OUTGOING: , smtp.hughes.net:465, SSL, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-6273da10-bd76-425a-a52c-9818c2150715 (7/15/2015)

 Extensions
   ImportExportTools, 3.2.4.1, false, {3ed8cc52-86fc-4613-9026-c1ef969da4c3}
   Mail Merge, 4.6.1, false, [email protected]
   Manually sort folders, 1.1.1, false, [email protected]

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     browser.zoom.full: true
     dom.ipc.plugins.contentTimeoutSecs: 30
     dom.ipc.plugins.hangUITimeoutSecs: 30
     dom.ipc.tabs.shutdownTimeoutSecs: 30
     extensions.lastAppVersion: 45.5.1
     font.internaluseonly.changed: true
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Arial
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Arial
     font.size.fixed.el: 14
     font.size.fixed.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.variable.el: 17
     font.size.variable.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     gfx.blacklist.suggested-driver-version: 6.1400.1000.5218
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 7b8e86ac-82c4-452b-9301-3a8b170b2ee
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     network.http.network-changed.timeout: 60
     network.http.spdy.ping-timeout: 60
     network.predictor.cleaned-up: true
     network.websocket.timeout.close: 60
     network.websocket.timeout.open: 30
     network.websocket.timeout.ping.response: 40
     places.database.lastMaintenance: 1482711417
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     plugin.state.flash: 0
     plugins.update.notifyUser: true
     print.printer_Canon_MX870_series_Printer.print_bgcolor: false
     print.printer_Canon_MX870_series_Printer.print_bgimages: false
     print.printer_Canon_MX870_series_Printer.print_colorspace:
     print.printer_Canon_MX870_series_Printer.print_command:
     print.printer_Canon_MX870_series_Printer.print_downloadfonts: false
     print.printer_Canon_MX870_series_Printer.print_duplex: 0
     print.printer_Canon_MX870_series_Printer.print_edge_bottom: 0
     print.printer_Canon_MX870_series_Printer.print_edge_left: 0
     print.printer_Canon_MX870_series_Printer.print_edge_right: 0
     print.printer_Canon_MX870_series_Printer.print_edge_top: 0
     print.printer_Canon_MX870_series_Printer.print_evenpages: true
     print.printer_Canon_MX870_series_Printer.print_footercenter:
     print.printer_Canon_MX870_series_Printer.print_footerleft: &PT
     print.printer_Canon_MX870_series_Printer.print_footerright: &D
     print.printer_Canon_MX870_series_Printer.print_headercenter:
     print.printer_Canon_MX870_series_Printer.print_headerleft: &T
     print.printer_Canon_MX870_series_Printer.print_headerright: &U
     print.printer_Canon_MX870_series_Printer.print_in_color: true
     print.printer_Canon_MX870_series_Printer.print_margin_bottom: 0.5
     print.printer_Canon_MX870_series_Printer.print_margin_left: 0.5
     print.printer_Canon_MX870_series_Printer.print_margin_right: 0.5
     print.printer_Canon_MX870_series_Printer.print_margin_top: 0.5
     print.printer_Canon_MX870_series_Printer.print_oddpages: true
     print.printer_Canon_MX870_series_Printer.print_orientation: 0
     print.printer_Canon_MX870_series_Printer.print_page_delay: 50
     print.printer_Canon_MX870_series_Printer.print_paper_data: 1
     print.printer_Canon_MX870_series_Printer.print_paper_height: 11.00
     print.printer_Canon_MX870_series_Printer.print_paper_name:
     print.printer_Canon_MX870_series_Printer.print_paper_size_type: 0
     print.printer_Canon_MX870_series_Printer.print_paper_size_unit: 0
     print.printer_Canon_MX870_series_Printer.print_paper_width: 8.50
     print.printer_Canon_MX870_series_Printer.print_plex_name:
     print.printer_Canon_MX870_series_Printer.print_resolution: 0
     print.printer_Canon_MX870_series_Printer.print_resolution_name:
     print.printer_Canon_MX870_series_Printer.print_reversed: false
     print.printer_Canon_MX870_series_Printer.print_scaling: 1.00
     print.printer_Canon_MX870_series_Printer.print_shrink_to_fit: true
     print.printer_Canon_MX870_series_Printer.print_to_file: false
     print.printer_Canon_MX870_series_Printer.print_unwriteable_margin_bottom: 0
     print.printer_Canon_MX870_series_Printer.print_unwriteable_margin_left: 0
     print.printer_Canon_MX870_series_Printer.print_unwriteable_margin_right: 0
     print.printer_Canon_MX870_series_Printer.print_unwriteable_margin_top: 0
     print.printer_Canon_MX920_series_Printer.print_bgcolor: false
     print.printer_Canon_MX920_series_Printer.print_bgimages: false
     print.printer_Canon_MX920_series_Printer.print_colorspace:
     print.printer_Canon_MX920_series_Printer.print_command:
     print.printer_Canon_MX920_series_Printer.print_downloadfonts: false
     print.printer_Canon_MX920_series_Printer.print_duplex: 0
     print.printer_Canon_MX920_series_Printer.print_edge_bottom: 0
     print.printer_Canon_MX920_series_Printer.print_edge_left: 0
     print.printer_Canon_MX920_series_Printer.print_edge_right: 0
     print.printer_Canon_MX920_series_Printer.print_edge_top: 0
     print.printer_Canon_MX920_series

It doesn't appear that the entire troubleshooting log was included in my last post. Here are the log entries after the printer documentation:

Graphics

     Adapter Description: Intel(R) HD Graphics 4400
     Vendor ID: 0x8086
     Device ID: 0x041e
     Adapter RAM: Unknown
     Adapter Drivers: igdumdim64 igd10iumd64 igd10iumd64 igdumdim32 igd10iumd32 igd10iumd32
     Driver Version: 9.18.10.3272
     Driver Date: 8-8-2013
     Direct2D Enabled: false
     DirectWrite Enabled: false (6.2.9200.21976)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 4400 Direct3D11 vs_5_0 ps_5_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.316930d51ea9)
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS Util
     3.21.3
     3.21.3

     NSS SSL
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS S/MIME
     3.21.3 Basic ECC
     3.21.3 Basic ECC

Pk, the error you are getting is not the one I would expect from those settings. Relay not permitted is the error I would expect.

Basically you are trying to send mail from smtp.hughes.net for accounts that are not provided by them. Normally the SMTP server simply rejects the send with a relay denied error, apparently Hughes just times out for some reason.

Why are you not using the registered mail exchanger for those domains? I know the godaddy service can be fairly poor, but your ISP it would appear is not going to play any more. You might get some joy in asking them, but I doubt it.

Hughes.net was used as smtp (years ago) because it was more reliable. At your suggestion, I've changed back to the mail.dynamicmediasolutions.us smtp setting for my email account and the results are improved but still unpredictable. If I'm patient enough (sometimes as many as five attempts) an email with a large attachment (2-5 mb) will eventually send. BTW, I can tell by the progress bar if it's going to go or not. Hughes.net is a slow boat and if the progress bar zips across to 100% within a few seconds, it's not going to send. If the progress bar gives status (51, 65, 72%, etc.) updates that last at least 10 seconds, the email will send. The other two dynamicmediasolutions.us workstations are happy with hughes.net smtp and any size attachment.

Not sure all that helps, but I don't have the time to keep fooling around with this issue and -- although the process is cumbersome -- I am able to send email from my workstation with large attachments (eventually). So, I think I'm ready to retire this thread unless you have other suggestions.