==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.6.2
- Deployed with iRedMail Easy or the downloadable installer? Downloadable
- Linux/BSD distribution name and version:  Debian 11
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MariaDB
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

When sending email to an iRedMail server, the server takes 30-60 seconds between initial setup of TCP and TLS connections over 587/tcp and login and message acceptance:

Feb 11 14:58:46 mail10 postfix/submission/smtpd[469621]: connect from cpe-X-X-X-X.socal.res.rr.com[X.X.X.X]
Feb 11 14:58:46 mail10 postfix/submission/smtpd[469621]: Anonymous TLS connection established from cpe-X-X-X-X.socal.res.rr.com[X.X.X.X]: TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256
Feb 11 14:59:22 mail10 postfix/submission/smtpd[469621]: 4PDmJB2NzZzHTbm: client=cpe-X-X-X-X.socal.res.rr.com[X.X.X.X], sasl_method=PLAIN, sasl_username=someuser@mydomain.com
Feb 11 15:00:22 mail10 postfix/submission/smtpd[469621]: disconnect from cpe-X-X-X-X.socal.res.rr.com[X.X.X.X] ehlo=2 starttls=1 auth=1 mail=1 rcpt=1 data=1 quit=1 commands=8

DNS works OK for both client and server.

Initially I thought this was happening because I'd added an AAAA record for the server, but that's not the problem. If I do a packet capture, the initial connection from the client uses the server's IPv4 address. Also, the problem persists even after I'd removed the AAAA record and waited longer than the record's TTL.

There was a previous thread on this topic, but it appears the root cause there was LDAP, and I'm not using LDAP:

https://forum.iredmail.org/search1953093385.html

Thanks in advance for clues on how to troubleshoot and fix this issue.