Has this ever worked for you?

It would seem that the SMTP server might be configured to work with a somewhat small number of addresses per message; this could be a klunky and lame anti-spammer precaution on the part of your email povider.

An alternative might be to try a mail merge, where you generate one separate message for each addressee, but if your SMTP is getting choked up on multiple addresses per message, firing a lot of messages at it in short order would probably have a similar effect. :-(

https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/

Zenos said

Has this ever worked for you? It would seem that the SMTP server might be configured to work with a somewhat small number of addresses per message; this could be a klunky and lame anti-spammer precaution on the part of your email povider. An alternative might be to try a mail merge, where you generate one separate message for each addressee, but if your SMTP is getting choked up on multiple addresses per message, firing a lot of messages at it in short order would probably have a similar effect. :-( https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/

The bulk send-out was working one day before. My server provider checked on their side - its OK, so it is something in Thunderbird

Well, it's a message from a server. That's not to deny that something has changed in Thunderbird to provoke this, but so far you're the first user to have reported this issue.

Just to be clear; is a "batch" just a lot of separate messages in succession, or is it triggered by using a mailing list, or a few messages with large numbers of addressees?

Zenos said

Well, it's a message from a server. That's not to deny that something has changed in Thunderbird to provoke this, but so far you're the first user to have reported this issue. Just to be clear; is a "batch" just a lot of separate messages in succession, or is it triggered by using a mailing list, or a few messages with large numbers of addressees?

Same email with a large number of addresses. Another problem was when adding, say 300, address to the BCC it causes Thundrbird to crash. Perhaps this helps to identify the problem?

re: I send serveral emails in one batch

Are you sending this using a created thunderbird 'Mailing list' or using addon 'Mail Merge' or just selecting several contacts?

Some servers set a limit on the number of emails you can send in a 'batch'. Other servers set a limit in the number of emails you can send in a given amount of time. What is ISP and server?