Rob (BCC), Bron (BCC), will you please make someone have a look at it?
The problem is that Fastmail MX respond with 4xx to addresses at domains
which are not configured at Fastmail AND reports each of those temporary
rejections as an attempt of delivery to non-existing user. This has made
my own tiny server get bad "sender score"... Now I don't know how much
sender score is really used and whether it will be reset at some point.
• Marco M. via mailop [2023-10-08 08:55]:
Am 07.10.2023 um 22:41:01 Uhr schrieb Kirill Miazine via mailop:
Recently I experienced a very "interesting" case which gave my
personal email server "poor" "sender score" due to high rate of
emails sent from my server to non-existent users.
Normal, because spammers often have mass of non-existing (mostly not
anymore) addresses and because these lists are often sold, many
spammers will try it.
I think I figured out what it was. A user sent an email to another
family member, but using contact's old email domain which used to be
hosted by Fastmail, but where no such user would exist anymore (I
don't know if domain still exists as a domain at Fastmail, but
clearly MX point to them).
Actually the error message "Relay access denied" indicates that domain
is not configured at Fastmail anymore. I just tried and non-existent
users give a different error message:
550 5.1.1 <does-not-exist@...>: Recipient address rejected: User unknown
in virtual mailbox table
I'd guess a 5xx response would be appropriate -- at least on port 25,
but Fastmail sent 451, and made my Exim try -- and keep re-trying --
all IP addresses of all their MX. I have very conservative retry
rules, so emails would be kept in the queue for a month...
That is fastmails bad configuration. Permanent errors like "mailbox
doesn't exist" must have a permanent error code (5xx) to avoid retry.
Senders to Fastmail beware, and maybe consider treating their 4xx
errors as permanent.
Fastmail must fix that - every MTA will try it again and again if a 4xx
error occurs and that will flood them with the messages many times.
And senders given bad sender score eventually, and especially low-volume
servers.
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