Probably a wrong space or another separator. I have a database which I expor tto CSV, open in LibreOffice Spreadsheet, edit, sort and then copy multiple rows of the e-mail column and paste them in Thunderbird.

It works.

Some cells have multiple e-mail addresses, the addresses are most of the time separated by a Comma. But sometimes someone puts a semi-colon instead and it prevents TB or the gmail server from sending.

Other issues arise when names have special characters and specially if they are in the format "John D. Doe <jd@gmail.com>" (no quotes) sometimes the name is split and it is then broken since John D isn't an e-mail address and Doe <jd@gmail.com> is, but the regular expression splitted that as two contacts.

Thanks, but that doesn't explain why the list sometimes is accepted and sometimes is not. It always works if it is the only address being used. FYI, each of my mailing lists is created within TB.

Is each 'mailing list' on a separate line in it's own 'To' field?

please post image showing the two mailing list names as they would normally appear in a new 'Write' message. You would probably have one saved in the 'Sent' folder.

How many contacts are in each mailing list ?

It is also possible that the two lists combined exceed some limits on number of emails that can be sent within a given period.

Does it matter which mailing list is the first one ? - in other words: Does it always fail on the second mailing list - not the first one - irrespective of whether you swap the order of the mailing lists in the 'TO' fields.

Thanks for your comments. Yes, each mailing list is on a separate line. Sometimes as a "TO" sometimes as a "bcc." The number of addresses within each list varies, but generally is a half dozen or so. Most often, it seems to happen with the last mailing list addressee, but that is not always the case.

Thanks for your assistance.