Group Mail Stopped 5.1.1 which, I suppose,refers to Not Our Customer Message

[global.citizen]

Version: 2004
Operating System: Mac OS X 10.5 (Leopard)
Processor: Intel

Often, when trying to send a message to a group, I get the message "5.1.1 Not Our Customer" which, I presume, refers to one or more of the recipients' addresses.

How can the mail be sent for the others?

 

[Bras]

This error may come up when you send to too many receipients, most ISP's
(Internet Service Provider's) have a limit of 50 receipients per e-mail and
a limit per day. Also check the addresses in your group, there may be one
with a space or a character that is not used in e-mail addresses.
Also it could be your ISP telling you that you are sending with a third
party e-mail address, if so then try to use authentication for the SMTP
server, this is found when you click the advanced sending options button
under the SMTP server in account settings.
You can also try to use the SMTP of your ISP but then you probably need
authentication with their username and password.

Bras