When adding a new GMail account an OAuth2 window pops up and everything works as expected. However the SMTP authentication settings have Authentication set to PLAIN, which does not work.

Setting this to XOAUTH2 solves the issue. So the bug seems that KMail seems to set PLAIN authentication by default even when an OAuth2 authentication method was used to add the account. 

How to manually fix the issue: Configure KMail -> Accounts -> Sending -> Select Google Mail SMTP -> Modify... -> Advanced -> Change "Authentication" from PLAIN to XOAUTH

STEPS TO REPRODUCE
1. Configure KMail -> Accounts -> Receiving -> Add... -> Add Mail account -> *follow wizard*
2. Try to send an email, which fires a warning email from google an does not work.

OBSERVED RESULT
Email does not send via SMTP because Google Mail does not let plain authentication to send emails by default.

EXPECTED RESULT
SMTP authenticates via OAuth2 by default if that what's been used to add the account, so sending mail works.


SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.2
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Kernel Version: 5.14.16-zen1-1-zen (64-bit)
Graphics Platform: X11
Processors: 8 × 11th Gen Intel® Core™ i7-1165G7 @ 2.80GHz
Memory: 15,4 GiB of RAM
Graphics Processor: Mesa Intel® Xe Graphics