I'm assuming you are using the 'Contact Sidebar' to highlight names and then click on the 'Addto Bcc' button. Please advise if otherwise.

I've just been testing this out in a number of methods and I'm not getting anything like what you say.

What version of Thunderbird are you using?

This could be an addon extension causing a conflict.

In Thunderbird

• Help > Troubleshooting information
• click on 'Copt text to clipboard' button
• In this forum question, right click in a 'Reply' text box and select 'Paste.
• You can remove everything with the exception of the 'Extensions' section.

Start Thunderbird in Safe Mode. Help > restart with add-ons disabled.

Test the same 17 people added as Bcc by the method I mentioned.

I think most of your questions will be answered below.

Here are the details of my Thunderbird application:

Version. 52.8.0, (User Agent, Mozilla /5.0 (Windows NT 10.0; WOW 64; rv:52.0) Gecko/20100101 Thunderbird/52/8/0. Profile Folder= Open Folder. Application Build ID; 20180516145259.

Mail and News Accounts: Incoming Server account 1: Name = (pop3) pop.verison.net995, Connection Security = SSL/TLS; Authentication method =Normal Password. Outgoing Server: Name = smtp.verizon.net.465, Connection security = SSL/TLS, Authentication method = Normal password.

Here is how I normally forward an email:

The email is open and I click on the "forward tab". Then I would be looking at Subject: Fwd: Name of Document. From: My Name and email address and To: with a flashing cursor. On the left side of the page is a column of contacts names and at the bottom of this list their are 3 choices: Add to To:, Add to Cc: and Add to Bcc:

I choose my first contact, then press down my Ctrl Key, and then choose all of the contacts that I want to receive a copy of the email. Then I click on the Add to Bcc: tab and all the selected contacts are shown in the address column as going to Bcc:

The problem is sometimes, not all the time, when I add an address, it is automatically placed in the To: category, I have not told it to do this, it just does it. If I have already selected other addresses before this happens, they are also assigned to the To: category. However, in the past I was not noticing this because I am just adding new addresses. Then, I am still adding addresses and when I get all addresses entered I click on the Add to Bcc: category and all of the addresses entered are placed in the Add to Bcc:. Then when I try to forward the emails I get the message "An error occured while sending mail. The mail server responded:5.5.0 Recipient already specified." What happens is that all the addresses that were placed in the To: category are also placed into the Bcc: category, and creating the problem of two addresses for some recipients.

Here is an example: I am adding addresses, I am holding down the Ctrl Key, and I have entered 6 addresses and there is no designation as to whether they are To:, Cc:, or Bcc:. When I enter the seventh address it and the prior six are shown with To: to the left side of them. That is all I have done. Now, I continue to enter more addresses and there is no designation on the left side of them. When I have entered all the addresses I click on the Add to Bcc:. The first 7 addresses will be shown as To: and Bcc:, those entered starting with number 8 are shown are only shown as Bcc: Most of the time I can delete this, redo it, and it will only show the Bcc: category. This is a very random procedure, sometimes it will change to To: on the third address and sometimes it will be #16 of a total of 18 entries.

Does anyone know why this happens, and how I may avoid it?

Application Basics

   Name: Thunderbird
   Version: 52.9.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180629153814
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.verizon.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.verizon.net:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-e55eca22-848f-42e5-90d6-037bd0180423 (4/23/2018)
   https://crash-stats.mozilla.com/report/index/bp-d363aa03-b7e7-410d-94d3-df7fa0180418 (4/17/2018)
   https://crash-stats.mozilla.com/report/index/bp-32e9064a-d3b3-43a6-a046-075180170504 (5/4/2017)

 Extensions
   Lightning, 5.4.9, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

Hi I've been testing the exact process you mention.

I can replicate the issue but only when I do one particular action. So I start exactly the same as you.... I am adding addresses, I am holding down the Ctrl Key, and I have selected/highlighted 6 addresses by using a single left click and as yet not selected designation as to whether they are To:, Cc:, or Bcc:.

However, at the point of entering the seventh address, I 'accidentaly on purpose' did a double click and all seven email addresses are instantly added in the TO fields.

You do not need to click on one of the 'Addto TO' buttons; a double click will auto do this for any preselected email addresses.

As this is occurring sometimes and intermittantly and can be four address or more or any number, this has to be initiated by a double click on the last inserted 'TO' email address. This would explain why it does not occur all the time or how different numbers of email addresses can be added.

Thanks for your help. We can always learn something new. I did not know that double clicking an address would automatically do this. I don't think I would have ever solved this on my own. Also, I didn't know that I was double clicking, that was a surprise. Now I'll have to be a little more attentive.

After trying the solution chosen, I was still having the same problem from time to time and I couldn't understand why. I was being very careful to not double click and it got to where I couldn't hardly get all addresses to be Bcc. So I started searching for another possible reason. I found out that sometimes the culprit will be your mouse causing this problem. I am using a Logitech mouse, and it seems that quite a few people were having this problem (double clicking when you only click once). Do a search for whatever mouse you're using and you will find this is not an uncommon problem. There are videos that show you how to repair a mouse, but I just replaced my mouse and now I don't have any problem at all, I am not having to be real careful when I click. This explained why I couldn't believe that I was double clicking. Hope this further helps anyone with this problem.