One of my customers is experiencing delayed mail delivery. Sometimes they won't get the message until the nextday.

This customer is using POP3 and SMTP on my server. This handles their interoffice mail so even sending to each other vs. other internet addresses they are having this issue.

No other sites are having this problem, but no other sites are using the box for SMTP.

Anybody have any ideas what could cause this...

Well, even if they are sending it out through your server, it has to go out through their ISP first (relayed) and maybe this is causing some problems. I'd suggest they just use their ISP's SMTP - see if it works better that way.

Except they have a leased t1 that does not include mail service that is why they are using my SMPT server. I have it restriced to their IP block provided by thier ISP. So that is not really an option.

So you don't think it would be anything on the box?

I know this may seem a little silly, but have you aked them how often they have their client set to send/receive mail? If user A composes mail and hits "send", which may actually be "que", and his client only connects every 2 hrs, the message wouldn't be sent until then...and then the same on the receive side.

I would probably set up a test account and see for yourself - you could send a mail to the account and then telnet in and use PINE to see if the mail is in the mailbox.

Mark<===has a knack for the obvious

that is a good thought, they are using outlook for their client and it is set to check for mail every 2 minutes. It only happen occasionally, not on every message. But when it does happen, it does not get delivered until the next day, weird huh...