There are three obvious options:
AppleShare IP 5 (for 7.6.1; version 6 for various later Systems)
Apple Internet Mail Server
Eudora Internet Mail Server
The latter two were freeware. ASIP can, however, be found if you go looking.
Claris also produced OfficeMail, but I am not sure how viable that would be to use (e.g., I think it likely required a modem connection). And I suppose that it would, in principle, be possible to experiment with a PowerTalk-based mail system in System 7.5, but I am not sure that you would get it to work on today's internet. It would, however, make an interesting retrochallenge project.
AppleShare IP 5 (for 7.6.1; version 6 for various later Systems)
Apple Internet Mail Server
Eudora Internet Mail Server
The latter two were freeware. ASIP can, however, be found if you go looking.
Claris also produced OfficeMail, but I am not sure how viable that would be to use (e.g., I think it likely required a modem connection). And I suppose that it would, in principle, be possible to experiment with a PowerTalk-based mail system in System 7.5, but I am not sure that you would get it to work on today's internet. It would, however, make an interesting retrochallenge project.