==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.2.1
- Deployed with iRedMail Easy or the downloadable installer? installer
- Linux/BSD distribution name and version: Ubuntu 18.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): remote LDAP server
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Yes 4.4
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
I read through this:
https://docs.iredmail.org/errors.html#r … der-domain

because I am getting an uncomfortable number of these:
Aug 17 02:21:10 mail01 postfix/smtpd[23180]: 4BVP7R4kTSz4J: reject: RCPT from mailhost.domain.com[69.24.99.99]: 554 5.7.1 <paulc@domain.info>: Recipient address rejected: SMTP AUTH is required for users under this sender domain; from=<system@domain.com> to=<paulc@domain.info> proto=ESMTP helo=<dfmx03.domain.com>

Does the IP Whitelisting in the settings.py support CIDR formatting?

I am going to implement the ip address whitelisting but was curious what is causing this. both domain.com and domain.info are hosted on the server. The email being sent was an email generated by a server and sent as system@domain.com which is a mail list on the iRedMail server. I have 20+ systems emailing this way and just once in a while an email gets rejected for SMTP auth. The system did not send the email via iRedMail but by another smtp server. All the IPs are whitelisted in the SPF record for both domains.
I will also try:
ALLOWED_LOGIN_MISMATCH_LIST_MEMBER = True
Even though the email wasn't sent using iRedMail.