Unfortunately the fallback configuration code for [mailer] that were
added in #18982
An error occurred
are incorrect. When you read a value from an ini section
that key is added. This leads to a failure of the fallback mechanism.
Further there is also a spelling mistake in the startTLS configuration.
This PR restructures the mailer code to first map the deprecated
settings on to the new ones - and then use ini.MapTo to map those on to
the struct with additional validation as necessary.
Ref #21744
zeripath