Had the same issue with my hosting company and Outlook for Windows AND Mail for Mac. As Dan said, Port 25 is often blocked due to spam. What happens is that certain ports block out certain emails.. I had ONE email that wouldn't send because it had the "PEN IS." Not like a schlong, but as in "The pen is..." (Ordering custom pens).
Changed the port to 115 (100-something) and everything works now!
From my knowledge, you can send email on one port - or the other but not both. Port 115 is usually reserved for sftp traffic - not smtp. The host could have very well setup the server to communicate using port 115 but then it would not accept mail on port 25 (which doesn't explain why he could send 5 previous messages). Could it be that some spam device thought he was spamming (from inside the network)? Possibly.
Maybe you mean port 143 (IMAP)? Even if this was the case, it doesn't explain how he 'could' send other emails.
Abbey, I agree with all the others, get back in touch with your ISP and go through all the specifics of the configuration.