@aaPanel_Jose
Please do take note the Mail Service 4.1 has a serious bug that existed since last 5 version of aaPanel release.
The bug can potential leads some of the email address that created via the Mail Service 4.1 is unable to deliver to the gmail.
Although during the setup process, the system has configured the proper hostname on the CentOS clean installation environment but when the Mail Service 4.1 installation, it does not actually clone the entire hostname that is specificed in the OS environment.
Please do make some necessary fixings to resolve this issues.
This error only occurs when you have a plain installation, existing environment will not be affected.