There is probably nothing wrong with the set-up. I'd suspect a timing issue because of the uncertainty as to how much time users will take to connect via SMTP (to send mail) after connecting to POP.

See http://en.wikipedia.org/wiki/POP_before_SMTP.

That's what I suspected too, but why just this 1 user?? I did have her login via the "Get Mail"process prior to sending, but that intermittently errors too..

Thanks, appreciate the feedback..

That 127.0.0.1 indicates a proxy of some sort (that being the adapter loop back address)

Try restarting the PC in windows safe mode with Networking. That will eliminate all of the residents security stuff and isolate if the issue is in the PC or elsewhere.

Note that if your using SSL or STARTTLS with the connection it is highly probable that the anti virus either ignores the data stream entirely or makes a mess of it trying to hack it. Or installs it's self as a proxy and makes adds the SSL/TLS and Thunderbird settings have none. This is all well while things work, but those proxies do not relay errors back very will in most cases.

Thanks Matt... I know that 127.0.0.1 was probably a patch point for the Avast email scanner, and have considered that it may be the issue, that's why I disabled it.. All the other users PC's are identically setup, (ie: same programs, same AV, etc., ).. they work flawlessly... The major problem with troubleshooting, is that the error only happens after Thunderbird hasn't sent any email for awhile, and even then it only fails occasionally.. Once it's working, you can send email after email w/o any errors??

Will definitely take your advise and try safe mode, to see if anything changes.. Appreciate the input...

any luck?

Won't have any access to the system till tomorrow, but just found out that it has actually happened on one of the other PC's once, last week... So, I'm starting to lean towards an intermittent timing issue.. I still plan on figuring out how to tell what program is using the loopback address as a patch point for intercepting email.. Any ideas?

That's the sort of thing process monitor from Sysinternals is good for. http://technet.microsoft.com/en-us/sysinternals

Matt, worked on it over a 4 hr. span, and was unable to get it to error out once?? Started at 30 min. between sends, and worked up to 2 hrs. and it worked fine?? The only difference was she has other programs running in the background, some of which, I am unable to use, due to financial privacy issues.. Odd thing is, I have been unable to locate any errors in the event viewer logs, that are indicative of this problem...

Thanks goodday, I'm familiar with process monitor, but not exactly sure how it would help in this case... Since no services are killed, and it only returns an error message, how would process monitor indicate that there was a problem??

Appreciate the help, guys... Hope to work on it more today, if I can..

I think you will have to use her software. My guess is it is some sort of banking "security" stuff. I have seen people have mail problems before when they run security rubbish from banks which makes the bank transaction secure at the expense of everything else.

You can use it to see network activity on 127.0.0.1

Sorry for responding so slowly, but due to lack of access and other issues, it just took more time to resolve than normal.. Changed from an unencrypted to encrypted mail server on the host, and changed the appropriate settings on Thunderbird, and problem was resolved, at least as long as the host can keep the encrypted server running, but that's another story!! So, for this user, it's the answer, but none of the other users have to use the encrypted mail server, and they experience no problems??? Thanks for all the help, everyone....