It's a convention in rails to name your mailers with a Mailer suffix, and is also common to name the class after the recipient (e.g. User, Admin). So UserMailer seems a reasonable choice.
If we delay sending failure notifications then they will fail
because the trace will no longer exist.
As trace imports are running in the background anyway there
doesn't seem to be any good reason to defer the emails.
Fixes#2312
