Use 587 as default SMTP port, fallback to 25 if 587 fails in ServerTest

Port 25 is generally used for MTA-MTA communication, while port 587 should be
used for email submission by MUAs (and major services like gmail don't even listen
on port 25 anymore). The ServerTest will still fall back to trying port 25 if
it fails to connect on port 587 to keep supporting anctient and misconfigured
SMTP servers.