==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.4
- Deployed with iRedMail Easy or the downloadable installer? downloadable installer
- Linux/BSD distribution name and version: CentOS 7
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  LDAP
- 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.
====

Hello,
we have a domain "domain.com" with one user on iRedMail. We don't want to use mail services for this domain (and its users), so we uncheck "Enabled Services: Mail service". This way we can still use the user for LDAP login for different services, but not use the user for email.
The domain "domain.com" is hosting on another mail server. We use the same domain and username so the user can login into application with the same email address.

The problem is if the user from real mail server sends an email to another user (seconddomain.com) on our mail server, we get an error:

REJECT SMTP AUTH is required for users under this sender domain

But if mail services are disabled, iRedMail should see that mail services are disabled and deliver the email normally to local account.
If problem is not clear, I can further explain and also give mail logs.

Thank you!
Slovenka