Switch from SMTP+STARTTLS on port 587 to SMTPS on port 465

A very good argument for using 465 is made at
https://www.agwa.name/blog/post/starttls_considered_harmful.

FastMail make a good case at
https://www.fastmail.fm/help/technical/ssltlsstarttls.html.

Gmail uses 465 too.

This shouldn't stop opportunistic TLS being used for incoming mail because
STARTTLS remains available on port 25 - which is what would have been used
before too.