At our office, we host our own Windows 2000
server, running "Imail". For about the last four
months, we've had problems where all of the
sudden we cannot send emails to addresses that
previously posed no problem. The error message
simply indicates that the emails are
"undeliverable".

I've talked to a friend, who lives in a different state,
and he tells me he is having the same problem.

Any insights or suggestions?

Did you call the CLient to see if they still using the same email address.

Yes. I know that the various addresses are good
addresses, and they have worked in the past; they
also seem to work when I send email from other
accounts (other ISP's and other servers).

Any suggestions? Thanks.