What prevents you from sending mail to those two contacts?

These are business accounts that I have corresponded with frequently for years. I can send to them, the messages go into the Sent Folder, but they never receive them if I use Thunderbird on one of my laptops. I am using Windows, not Apple. They do receive messages if I use my phone or the att.net site.

They checked their Spam and Trash and they do not appear there. From my end, they appear to have been sent.

Two issues to check.

The actual address. If Thunderbird has somehow managed to save the address with apace before or after then failure will occur and it will not be obvious.

SPAM filters at the receiving end might simply be dropping the emails.

Try sending the individual concerned a plain text email from Thunderbird.

• Hold shift when you click the write button to make it plain text
• When Thunderbird's write window opens you sill not have the usual formatting bar in the compose windows for font names etc.
• Delete your signature if it appear in the email (the most common cause of false spam identification)
• Now type a simple text message asking for a reply if received from your contact. now images of any sort or the message will not be sent as plain text.
• Use the format menu to specify the "delivery format" as plain text only and click send

Doing this will perhaps help to identify the cause as plain text is about a visually boring as you can get, but it is also about as innocuous as a mail can be made to look.

If plain text is not delivered, you may have to take the issue to your mail provider or the mail administrator at the receiving end. At time we see home IP addresses, but small business mail servers on shared machines and IP addresses getting black balled by anti spam lists because of their location in the IP range being a "bad" neighborhood. For example spamhaus which is used by many mail server checks if the sending server is on a "residential" IP address etc.

I did everything you suggested with no success when sending from Thunderbird from either of my laptops. I was successful when sending from the att.net site.

I believe that verifies that my email address is not blocked. Rather something in Thunderbird is affecting this.

I deleted the contacts and re-entered them with a slightly different name. Sending was not successful. i don't understand what else could be causing this problem.

The suggestion was that your IP address (or, more likely, the range of addresses to which it belongs) is blacklisted, not that your email address is blocked. Messages sent from Thunderbird will include your site's IP address whereas messages sent via webmail will have AT&T's IP address. Yours is very much more likely to be blocked.

My IP address was being blocked and the AT&T postmaster confirmed that my email account was set-up incorrectly as dynamic rather than static. They just fixed it. I can now send to the two accounts that were blocked.

Thanks for the help.