That is a message from your mail provider that you have been a bad boy and they have put you in timeout. Why? I have no idea. Perhaps ask them.

I would hazard a guess, as it clears quickly, that you are sending to much mail to comply with their terms of service so they are enforcing their TOS but that is only a guess.

That is a message from your mail provider that you have been a bad boy and they have put you in timeout. Why? I have no idea. Perhaps ask them. I would hazard a guess, as it clears quickly, that you are sending to much mail to comply with their terms of service so they are enforcing their TOS but that is only a guess.

As clearly as I could state it, at the very same time that this happens, I am able to send the same emails to the same recipients from the mail provider's service directly, so it is logical to assume it is not a restriction imposed from the mail provider, am I right in thinking so?

Matt said

That is a message from your mail provider that you have been a bad boy and they have put you in timeout. Why? I have no idea. Perhaps ask them. I would hazard a guess, as it clears quickly, that you are sending to much mail to comply with their terms of service so they are enforcing their TOS but that is only a guess.

As clearly as I could state it, at the very same time that this happens, I am able to send the same emails to the same recipients from the mail provider's service directly, so it is logical to assume it is not a restriction imposed from the mail provider, am I right in thinking so?

Unfortunately folk think that using their providers website to send mail offers some diagnostic value. It does not. Providers are very careful with their webmail, they have convinced more than 80% of their customers to use these services and usually get a load of advertising at the same time. For the remaining 20% the server used to send mail is rarely if ever the same one used to send mail from the website. This is compounded by many having completely different rules about mail volumes and sizes for SMTP V's Webmail. With the SMTP rout always having the most restrictive offerings and rules.

To reiterate, this is coming from the SMTP server. I have no information that would indicate if you are using even the correct SMTP server. I often see folk trying to send email using a Gmail SMTP server for an email address that has nothing to do with gmail or Google. They have weird issues as well.

To quote the message "the mail server said" that actually says the message originated with the server you have connected to. By the very nature of the message it is not coming from Thunderbird, despite it being the delivery mechanism. Ask your provider what the problem is, and when they say you can send from webmail so everything is Ok disagree with them and insist they support their SMTP server messages.

I did do a google search on the actual error badmailFrom list and that would indicate that the email address of mail server is marked as sending spam by the recipient server. That is the server at abv.bg is refusing delivery. But irrespective if it is your mail provider of your recipients, that does not change the fact that it is not an error that Thunderbird is issuing, it is one it to being given to deliver.

Yeh, contact your ISP and first question should be what port settings you should be using!