Use the default SMTP port of 25, not the MSA port of 587.

Horde advertises the default as 25 in the mailer configuration,
so this needs to match what the mailer code actually uses as the default.
If we want to change this, it needs to be changed as the advertised default
in the config and announced loudly in UPGRADING. Just spend 40 minutes
trying to figure out why I couldn't send mail after upgrading my checkout.