re :2) Any emails that are sitting in this troubled email account do not show up in the unified inbox, but the other two email accounts' inbox emails show up in the unified inbox

• Right click on the Unified 'Inbox' and select 'Properties'
• Click on 'choose'

All the Mail accounts will be listed will all the folders.

• You need to check that the 'Inbox' for each mail account has been selected.
• Click on OK
• Click on 'Update'

Select the Unified Inbox folder

re : 1) Whenever I send an email from this one email account (Comcast POP3 if it matters), I get the following error message: Sending of the message failed. An error occurred while sending mail. The mail server responded: Please ensure your email address and authenticating user match when sending messages. Please verify that your email address is correct in your account settings and try again.

Sounds like that account is trying to use the wrong outgoing smtp OR Xfinity does not have 3rd party email clients enabled.

Logon to webmail account and check to see if allow access to third-party programs in the Xfinity Email website is enabled . Use this info:

• https://www.xfinity.com/support/articles/third-party-email-access

If it needed switching on then it might fix the problem.

Otherwise check your settings because error says "Please ensure your email address and authenticating user match when sending messages" That implies your 'From' might says correct comcast email address, but that's not the email address being used in the server settings.

• Menu icon > Account Settings
• Left Pane - near the bottom below Local Folders - locate and click on 'Outgoing Server (SMTP)'

On the right it will display a list of outgoing servers. There should be one for each mail account. click on each one in turn and it displays info below You need to locate/select the one that has 'User Name' = full email address - same as the problem account.

• Then click on 'Edit'

If you cannot find one then you need to create one - click on 'Add'

Whether you are editing or adding - use the following info: Under 'Description' : please type the full email address of the account - note this should be the same address as shown in 'User Name'

You should have these settings:

• DEscription : full comcast email address - same as User Name
• Server Name: smtp.comcast.net
• Port: 587
• Connection Security : STARTTLS
• Authentication Method: Normal Password
• User Name: full comcast email address

• Click on 'OK'

Now check that comcast mail account is using that smtp.

• In Left Pane select name of problem comcast account
• Bottom Right you will which 'Outgoing Server (SMTP) server it is using.
• DO NOT click on 'Edit SMTP_Server' button - leave it alone.
• Click on the drop down option and select the one that shows the email address of same account.

Now Exit Thunderbird and allow a few moments for background processes to complete. Start Thunderbird.

Toad-Hall

Thanks so much for taking the time to help me! Both of your solutions worked perfectly!!