Using Notes Clients to POP3 to our Domino server

I'm using Notes Client 5.0.4a to SMTP to our Notes Server (5.0.3). 
Sometimes there is no problem, everything goes without a hitch.  Most 
times, however, I get the following on my Replicator screen:

Send outgoing Internet mail: No response from server

or sometimes:

Send outgoing Internet mail: Transient error received from SMTP host

There's no telling when I'll get these errors, or when it will clear 
itself up.  If I just keep trying it'll clear up sometime during the day, 
or maybe a couple of days later.  If I delete the Internet mail account, 
and re-create it, sometimes it'll clear up the message, most times not.

Any ideas?
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