@argv_minus_one
> the weird authentication protocol required by Gmail.
For that, specifically, I can only recommend: If a mail provider refuses to conform to standard auth protocols, they're a lost cause, ditch that provider.
As for this:
> what about email clients?
#Mutt has served my needs for decades, #AERC is worth a look: https://aerc-mail.org/
and, for mobile devices, #K9Mail: https://f-droid.org/packages/com.fsck.k9/