most mail providers impose a limit, their own choice of what they think is "to many". Your mail will have gone to no one.

thanks for the response Matt - presumably it's not a setting in Thunderbird that one can change. I looked for it without finding anything

Here is an article on Thunderbird limits. It seems to think there is a limit of 60 addresses if each one is entered separately. I do not know if these limits are still valid on current versions.http://kb.mozillazine.org/Limits_-_Thunderbird

@AirMail They probably are Airmail.... the array of addresses will be limited to 64K (2 to the power of 16) because 20 years ago the default allocation unit for C++ was 16 bits. The corollary is no one has seen a pressing need to change it as Mailing lists are a preferable way of multiple addressing anyway.

The article also mentions the 2 and 4Gb limits on folders. That has been history on IMAP accounts for years. It will also see it's final death knell in 38 with the last bug being fixed in the last 12months to enable greater than 4Gb files. The 2Gb limits for older Linux systems will continue as that is a limitation of the filing system used, not Thunderbird.

It is also noteworthy that the venerable MBOX will become optional in Version 38 with individual mail in their own files. This may be a boon for some and will certainly remove any limit on the size of a folder.

@Martin Your assumption is correct. Thunderbird does not even know what limits your particular provider has imposed. This can be a low of 50 at Comcast to unlimited. I have not seen less that 50. Gmail limits you to 100 from a mail client or 500 from their web site (capricious limits abound) This site lists some of the more popular providers and their respective limits. http://www.massmailsoftware.com/blog/smtp-email-sending-limits-and-rates-for-hosting-providers-and-free-email-providers/