intermittant sending of email via thunderbird

When sending via thunderbird for “group recipients” does not connect but sends me message:

“The message could not be sent because connecting to SMTP server name failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator”.

My settings are correct as when send “one” recipient delivers okay, this has me baffled as to why this should occur however my “Hotmail” works okay.

Appreciate any help on how to fix!!

Rgds,

John