I can send emails to any other address ini this domain but i have just created a new distribution group which, when I send an external email to I get the error:
The following recipient(s) could not be reached:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<#5.5.0 smtp;550 Invalid recipient <> (#5.1.1)>
What puzzles me is why this only happens on new distribution groups? The exhange server is not set to do a reverse DNS lookp and the distribution group works internaly. It may be worth mentioning that exchange is behind ISA firewall.