I run postfix 3.4.5.
I typically reject on unknown reverse hostname; it's a policy I'm comfortable
with.
For a number of correspondents that use outlook.com for outbound, I
occasionally see failures crop up for the same sender, then just
'automagically' resolve.
E.g., for a single sender, here "them @theirdomain.com", in my logs
postfix.log:Apr 24 13:18:19 mx postfix/postscreen-internal/smtpd[6816]:
NOQUEUE: client=mail-eopbgr770049.outbound.protection.outlook.com[40.107.77.49]
postfix.log:Apr 26 11:15:00 mx
postfix/postscreen-internal/smtpd[18428]: NOQUEUE:
client=mail-eopbgr790080.outbound.protection.outlook.com[40.107.79.80]
postfix.log:May 4 22:45:01 mx postfix/postscreen-internal/smtpd[6790]:
NOQUEUE: client=mail-eopbgr790083.outbound.protection.outlook.com[40.107.79.83]
postfix.log:May 6 08:22:44 mx
postfix/postscreen-internal/smtpd[45305]: NOQUEUE:
client=mail-eopbgr680058.outbound.protection.outlook.com[40.107.68.58]
postfix.log:May 9 10:19:32 mx
postfix/postscreen-internal/smtpd[51399]: NOQUEUE:
client=mail-eopbgr810078.outbound.protection.outlook.com[40.107.81.78]
postfix.log:May 16 11:36:11 mx
postfix/postscreen-internal/smtpd[19036]: NOQUEUE:
client=mail-eopbgr790048.outbound.protection.outlook.com[40.107.79.48]
?? postfix.log:May 17 9:41:37 mx postfix/postscreen-internal/smtpd[58594]:
NOQUEUE: reject: RCPT from unknown[40.107.72.40]: 550 5.7.1 Client host
rejected: cannot find your reverse hostname, [40.107.72.40];
from=<[email protected]> to=<[email protected]> proto=ESMTP
helo=<NAM05-CO1-obe.outbound.protection.outlook.com>
?? postfix.log:May 17 10:22:22 mx
postfix/postscreen-internal/smtpd[58594]: NOQUEUE: reject: RCPT from
unknown[40.107.81.79]: 550 5.7.1 Client host rejected: cannot find your reverse
hostname, [40.107.81.79]; from=<[email protected]> to=<[email protected]>
proto=ESMTP helo=<NAM01-BY2-obe.outbound.protection.outlook.com>
?? postfix.log:May 17 13:19:52 mx
postfix/postscreen-internal/smtpd[58644]: NOQUEUE: reject: RCPT from
unknown[40.107.82.59]: 550 5.7.1 Client host rejected: cannot find your reverse
hostname, [40.107.82.59]; from=<[email protected]> to=<[email protected]>
proto=ESMTP helo=<NAM01-SN1-obe.outbound.protection.outlook.com>
postfix.log:May 18 22:14:14 mx
postfix/postscreen-internal/smtpd[42996]: NOQUEUE:
client=mail-eopbgr770125.outbound.protection.outlook.com[40.107.77.125]
postfix.log:May 19 08:09:01 mx
postfix/postscreen-internal/smtpd[18025]: NOQUEUE:
client=mail-eopbgr80095.outbound.protection.outlook.com[40.107.8.95]
Checking around those fail times, there's no other legit/expected messages
rejected for reverse hostname; as such, I'm assuming that there's no blip in my
_own_ DNS resolver.
It seems to my this is possibly Microsoft/Outlook-specific ...
What's a reasonable way to make this more resilient -- without completely
disabling reverse hostname checks?
A DNS re-check before reject?
A whitelist for outlook.com?
