Nowdays client-server and server-server communication is ecrypted and signed, so no an issue now.
This is probably true, but in a very unsatisfying way. It’s not accurate to say this is not an issue now because mail servers talk to each other with opportunistic encryption — if both ends say “hey, I support TLS” they’ll talk over TLS, but if either end claims to not support TLS they’ll default to plain text. This is deeply concerning because it’s very possible for somebody to mimic another server and get the connection downgraded to plain text, bypassing TLS altogether. There are standards to deal with this, like DANE, but most large e-mail providers don’t support this… The other more recent standard to address this is called MTA-STS, but it’s much weaker than DANE and can potentially be exploited (but at least gmail and outlook support it, I guess). E-mail security is in a weird place. It’s slightly better than the “completely unencrypted” situation that people seem to think it is… But it’s also pretty much impossible to guarantee that your e-mail will not be sent over plain text.