Regardless of what mail client you use you'll have a small issue on your hands.
That issue is if the students are sharing the computers and each student has their own mail account AND you want them to have the freedom of accessing their mail from any single computer you have - what about their mail being spread throughout every computer they go to over time?
Unless your students are highly disciplined in never saving any email (always deleting) - this could get quite troublesome.
Example: I log into system 1 on Monday retrieve 4 emails (which save to system 1's HD).
Tuesday comes along, I log into system 2 to get mail this time.
Now I have some of my mail items on system 1 and some on system 2.
Obviously the way to go is to set up your clients to save the mail to the server and not the local system's HD. This is more based on what your mail server does than anything, and I don't know "sendmail" all that well.
I know you can do this with MS Exchange.
At any rate - pick any mail client - the client doesn't matter all that much to be honest. The only thing that REALLY matters is that it supports standard mail protocols - SMTP, POP3, etc. (Of course it matters that the protocols the client supports are also supported by the server).
In your case, with Win98 systems I'd say just give Outlook Express a try - its free (included with OS intially and you can get it with Internet Explorer).
If you don't like that - check out Eudora (I forget what version they are up to now). Eudora is very good IMHO.
Too bad you have your mail server already...I'd suggest IMAIL - which supports web messaging, which requires NO mail client at all! (You email through the browser).
WE