This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.
In some cases, the identifiers were un-exported after noticing they had
no external users.
Besides improving documentation, it also reduces the linter noise
significantly.
This patch is the result of running go vet, go fmt -s and the linter,
and fixing some of the things they noted/suggested.
There shouldn't be any significant logic changes, it's mostly
readability improvements.
This patch makes the SMTP courier distinguish permanent errors, so the queue
can decide whether to retry or not.
It's based on the SMTP replies: 5xy is considerent permanent, anything else is
transient (including errors other than protocol issues).
This patch adds SMTPUTF8 to the SMTP courier.
It introduces a new internal/smtp package that extends Go's net/smtp with
SMTPUTF8 (in a very narrow way, not for general use), and then makes the
courier use it.
Also use an IDNA-safe version when saying hello, otherwise servers could
complain if the hostname is not ASCII, and at that stage we don't know if they
support it or not.
