Have you tried contacting your mail provider? is you have the issue with tw mail applications (Desktop and Phone) then it is not something they are doing. It may be your mail provider is blacklisted as a spammer on one of the blacklists.

Some of my emails are NOTbeing received by others when I send them via Thurderbird from my desktop. When sending them from phone or from basic webmail program, they are getting through. I simply cannot reliably use thunderbird anymore unless the problem is fixed. The mail provider just says "everything is working via our webmail, so we have no problem to follow up on".

rbhansen said

The mail provider just says "everything is working via our webmail, so we have no problem to follow up on".

If you are prepared to accept the word of a script reading phone center worker, you might believe that rot. Personally I know better. In short if you want answers that actually make sense in the real world you have to make it to level two support and often you need to escalate your issue again from there before you actually get someone in the business that actually know the difference between a computer and a boat.

Seriously about the worst technical support in the world is provided by the call centers offered by the major ISP's. The staff have no training and basically no idea.

When Thunderbird sends mail, it puts up that little box saying sending mail and it leaves that box exactly where it is, blocking you doing anything else until your mail providers server says it has received the message for transmission, without error. So I matter not to me ehat you do, but it is without doubt that the folk that can offer your help in tracing what is happening to your mail is your mail provider, but only if you push them. They would prefer to tell you is is your copy of outlook and say good bye. (they told me that once and I did not have outlook installed.)

However I notice you are asking for help but have not een identified the mail provider that is lying to you.