...besides the obvious?
We were receiving the following error when our corporate mail server tried to send mail to any @shaw.ca address.
SMTP error from remote mail server after initial connection:
host idcmail.shaw.ca [220.127.116.11]: 550 Too many invalid recipients
We checked the usual suspects - made sure we were not accidentally on any black list, checked to see if we had done any mass mailings recently (we hadn't) and triple-checked the spelling of the addresses we were sending to.
Hotmail and Gmail -> Shaw worked.
Corporate mail server -> Hotmail and Gmail worked.
The problem went away while I was on hold with Shaw so we didn't get a chance to properly troubleshoot it. If anyone has some suggestions for what I should do next time, I would appreciate it.