Are you talking about sending many separate emails at once, or one email with many to: addresses in it? If the former, your email provider could be limiting you in order to discourage spamming. If the latter, this article might be relevant:

https://kb.mozillazine.org/Limits_-_Thunderbird#E-mail_addresses

"There appears to be a limit of approximately 60 addresses when sending messages if you enter each address separately. However, if you use lists (mail distribution lists) for most of the addresses any limit is imposed by whatever SMTP server you use."

No that is not the reason only have 15. Think i Need a detailed set of Instructions in making up and where to store list. May be an unrelated setting that is stopping them leaving. Used to work 100% once. maybe some up date or my unrelated setting. (dont know) obviously. Thanks for taking the time to reply. JOhn

If your issue is to create a mailing list, then do this: - click to addressbook pane - click the small icon to the right of 'new contact' and this opens a menu to create a mailing list from within the highlighted addressbook. = then, when sending message, use the mailing list name as the recipient.