Eudora 3.1.3L is a very good email client that works back then.
https://68kmla.org/bb/index.php?threads/networking-color-classic-to-modern-macos-x.36407/
explains that you can add an ethernet card to your color Classic.
Once you have an ethernet card, you can run an direct cable to your high speed modem or router, and just use OpenTransport to modify your TCP/IP settings.
I highly recommend upgrading to System 7.6.1. OpenTransport was not very stable under system 7.5.5. And by System 7.6.1 MacTCP was completely replaced.
IMAP Email is preferable over POP, because otherwise all your email will go directly to your Color Classic, and then you'll have to find a way to export it back to other machines.