[tildeteam] mailserver updates

hey ~teammates,

if you were unaware, i've switched from spamassassin to rspamd for our spam filtering needs.

it's more efficient and is able to filter that was sent to list addresses (i've been getting plenty of bounced list messages from non-members lately).

i also updated the default dovecot sieve rule to file any messages flagged as spam by rspamd into your junk folder.

please let me know if something isn't working or if you're still receiving any spam.

~ben