Postfix fails sending some mails

Hi to all!
Since i have upgraded my server from ubuntu server 22.04 to 24.04 i'm experiencing a curious behaviour from Postfix server.
I'm trying to send an email from my thunderbird client and sometimes it fails telling "destination domain not found (with some info more about the address...)" then i clic on "Accept", wait between 5 or 10 seconds, retry to send again and the mail is sent succesfully.
Never fails with gmail accounts...

I'm using pfsense as a router (fyi)

Code:

2026-05-18T22:28:14.321457+02:00 amserver postfix/qmgr[2436]: 18CC211E2B16: removed
2026-05-18T22:28:58.548830+02:00 amserver postfix/submission/smtpd[13668]: connect from unknown[170.253.55.27]
2026-05-18T22:28:58.857067+02:00 amserver postfix/submission/smtpd[13668]: NOQUEUE: filter: RCPT from unknown[170.253.55.27]: <[email protected]>: Sender address triggers FILTER lmtp:[127.0.0.1]:10026; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.100.155]>
2026-05-18T22:28:59.781619+02:00 amserver postfix/submission/smtpd[13668]: NOQUEUE: reject: RCPT from unknown[170.253.55.27]: 450 4.1.2 <[email protected]>: Recipient address rejected: Domain not found; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.100.155]>
2026-05-18T22:28:59.813895+02:00 amserver postfix/submission/smtpd[13668]: disconnect from unknown[170.253.55.27] ehlo=2 starttls=1 auth=1 mail=1 rcpt=0/1 quit=1 commands=6/7
2026-05-18T22:28:59.840111+02:00 amserver postfix/submission/smtpd[13668]: connect from unknown[170.253.55.27]
2026-05-18T22:29:00.098225+02:00 amserver postfix/submission/smtpd[13668]: NOQUEUE: filter: RCPT from unknown[170.253.55.27]: <[email protected]>: Sender address triggers FILTER lmtp:[127.0.0.1]:10026; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.100.155]>
2026-05-18T22:29:00.104929+02:00 amserver postfix/submission/smtpd[13668]: NOQUEUE: reject: RCPT from unknown[170.253.55.27]: 450 4.1.2 <[email protected]>: Recipient address rejected: Domain not found; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.100.155]>
2026-05-18T22:29:10.841344+02:00 amserver postfix/submission/smtpd[13668]: lost connection after RCPT from unknown[170.253.55.27]
2026-05-18T22:29:10.841727+02:00 amserver postfix/submission/smtpd[13668]: disconnect from unknown[170.253.55.27] ehlo=2 starttls=1 auth=1 mail=1 rcpt=0/1 commands=5/6
2026-05-18T22:29:12.533711+02:00 amserver postfix/submission/smtpd[13668]: connect from unknown[170.253.55.27]
2026-05-18T22:29:12.834111+02:00 amserver postfix/submission/smtpd[13668]: NOQUEUE: filter: RCPT from unknown[170.253.55.27]: <[email protected]>: Sender address triggers FILTER lmtp:[127.0.0.1]:10026; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.100.155]>
2026-05-18T22:29:12.860031+02:00 amserver postfix/submission/smtpd[13668]: D1DCA11E1D2C: client=unknown[170.253.55.27], sasl_method=PLAIN, [email protected]
2026-05-18T22:29:12.922890+02:00 amserver postfix/cleanup[13371]: D1DCA11E1D2C: message-id=<[email protected]>
2026-05-18T22:29:12.937575+02:00 amserver postfix/qmgr[2436]: D1DCA11E1D2C: from=<[email protected]>, size=612, nrcpt=1 (queue active)
2026-05-18T22:29:13.118507+02:00 amserver dovecot: imap-login: Login: user=<[email protected]>, method=PLAIN, rip=170.253.55.27, lip=192.168.71.4, mpid=13708, TLS, session=<mCZpah1SKtqq/Tcb>
2026-05-18T22:29:13.325033+02:00 amserver postfix/smtpd[13709]: connect from localhost[127.0.0.1]
2026-05-18T22:29:13.336607+02:00 amserver postfix/smtpd[13709]: 521B911E2B16: client=localhost[127.0.0.1]
2026-05-18T22:29:13.343238+02:00 amserver postfix/cleanup[13371]: 521B911E2B16: message-id=<[email protected]>
2026-05-18T22:29:13.355767+02:00 amserver postfix/smtpd[13709]: disconnect from localhost[127.0.0.1] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
2026-05-18T22:29:13.356012+02:00 amserver postfix/qmgr[2436]: 521B911E2B16: from=<[email protected]>, size=1753, nrcpt=1 (queue active)
2026-05-18T22:29:13.365712+02:00 amserver amavis[2812]: (02812-04) Passed CLEAN {RelayedOutbound}, ORIGINATING/MYNETS LOCAL [127.0.0.1] [170.253.55.27] <[email protected]> -> <[email protected]>, Message-ID: <[email protected]>, mail_id: Vcxof4SWVhyZ, Hits: -1, size: 612, queued_as: 521B911E2B16, dkim_new=default:airmonkey.es, 425 ms
2026-05-18T22:29:13.367246+02:00 amserver dovecot: imap([email protected])<13708><mCZpah1SKtqq/Tcb>: Disconnected: Logged out in=462 out=619 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0
2026-05-18T22:29:13.368647+02:00 amserver postfix/lmtp[13372]: D1DCA11E1D2C: to=<[email protected]>, relay=127.0.0.1[127.0.0.1]:10026, delay=0.57, delays=0.14/0/0/0.43, dsn=2.0.0, status=sent (250 2.0.0 from MTA(smtp:[127.0.0.1]:10027): 250 2.0.0 Ok: queued as 521B911E2B16)
2026-05-18T22:29:13.372302+02:00 amserver postfix/qmgr[2436]: D1DCA11E1D2C: removed
2026-05-18T22:29:16.104430+02:00 amserver postfix/smtp[13710]: 521B911E2B16: to=<[email protected]>, relay=mxext.gva.es[195.77.20.28]:25, delay=2.8, delays=0.02/0.05/1.3/1.4, dsn=2.0.0, status=sent (250 ok:  Message 26826058 accepted)
2026-05-18T22:29:16.105069+02:00 amserver postfix/qmgr[2436]: 521B911E2B16: removed

Here is the Postfix log where you can see (in the first lines the fail sending, and then when i retry, it sends the mail...

Any idea?

Many thanks,

Abraham.

 

Thanks for your answer Till.
I have configured pfsense as a DNS Resolver and i have 8.8.8.8, 1.1.1.1, 8.8.4.4 as a DNS's, and the server have the router IP as a DNS.
Is the same config before upgrade ubuntu from 22.04 to 24.04, and before the upgrade i have no problem with Postfix.
As i have no errors in the logs more than the error mentioned, i don't know where i have to look for the problem...
Thanks,
Abraham.