We are running exchange5.5 with SP4, we use mailsweeper4.3 as our SMTP gateway. sometimes on certain emails with attachments, the emails queue up in the outbound queue on the IMS with the decription "Host unreachable", I tried to hotmail.com and other domains using the same email message and got the same error. the message doesnt even seems to be to mailsweeper, so not sure whats the problem with these certain emails. can anyone advice ?