Classic-era email clients no longer support the security standards required by modern email services.
Your best bets are to run a local mail rely or host your email locally so you can choose what security standards to employ. You will not be able to get your yahoo or gmail accounts into a vintage mac email client, or probably even load their web pages.
Somewhat relatedly:
I'm working on this particular problem as part of the vtools project, (
forum thread,
web site,
patreon) - the intent there is that patreon members at the USD $5 per month support level will get home folders, email, and web hosting. The email addresses on this machine would be [username]@vtools.68kmla.org
The email function isn't available yet, and probably won't be until I can get some time (which, which COVID19 response at my university, and my being important/essential personnel for, might not happen for the near future) to replace my network gateway and migrate the data and users to the new box, after some more testing with aforementioned new box. However, PM me and I can create you an account on the existing machine, if you'd like to poke around at the software on it.
As part of moving to mac os x 10.4 server, there will probably be a webmail component. Sending will require authentication, which does limit things a bit to system 7/8/9-era stuff, a possible compromise is a VPN router that connects vintage Macs directly to the same network as vtools for unauthenticated access to SMTP, I'd have to check into what's reasonable and how to set that up and that would be something that might be available
far in the future.
EDIT/Addition: you could forward yuor main email account into a vintage-compatible email server, but to be honest I do not recommend it. No Classic Mac OS email client will display modern email formatting well. At this point (this
will change in the future) you pretty much need Mac OS X for that, and even that's kind of up-in-the-air.