I have installed Raspbian Buster with desktop on my Pi3B+ and am processing to install my apps. This is proceeding, most work allowing for newer versions, however I can't seem to send mail using ssmtp.
I installed ssmtp mailutils and configured /etc/ssmtp/revaliases and /etc/ssmtp/ssmtp.conf (as below) and ran sudo adduser pi mail
The version of ssmtp seems to be identical to that on Stretch, and I can successfully send from my Pi3A+ running Stretch, but not Buster.
I can logon to Gmail from chromium on Buster.
/etc/ssmtp/revaliases
# sSMTP aliases
#
# Format: local_account:outgoing_address:mailhub
#
# Example: root:your_login@your.domain:mailhub.your.domain[:port]
# where [:port] is an optional port number that defaults to 25.
root:XXXXXXXX.raspberry.pi@gmail.com:smtp.gmail.com:587
pi:XXXXXXXX.raspberry.pi@gmail.com:smtp.gmail.com:587
.
/etc/ssmtp/ssmtp.conf
#
# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
root=XXXXXXXX.raspberry.pi@gmail.com
# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
mailhub=smtp.gmail.com:587
AuthUser=XXXXXXXX.raspberry.pi@gmail.com
AuthPass=XXXXXXXX%XXXXXXXX
AuthMethod=LOGIN
UseTLS=YES
UseSTARTTLS=YES
# Where will the mail seem to come from?
rewriteDomain=gmail.com
# The full hostname
#hostname=
# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address
FromLineOverride=YES
/var/log/mail.err shows
Jun 26 15:04:57 MilliwaysPi3Plus sSMTP[1932]: (MilliwaysPi3Plus)
and dead.letter shows the contents of the email