Mail questions

[J]

I just experienced a rather strange phenomenon, or maybe it isn't that 
strange.

Instead of using Mail.app I signed on to this email address with my 
laptop using OS 9 and OE. I downloaded email that had been sitting on 
the server that I should have gotten weeks ago, from people who claimed 
they had answered me all along..
Apparently this mail program failed to retrieve. It has been picking up 
other mail regularly, and all accounts are active.

Is this common and what is the cure?

I did  notice the other day, that one of my accounts indicated that it 
had a 1 message in it, but the 1 suddenly disappeared after the 
retrieval stopped.

Also, one account continually gives me the "cannot retrieve mail on 
port 110" error.

Any ideas?

Thanks.