Mail server does not send e-mail from the virtual domain

Hello everyone

I am really new to the configuration of Webmin and linux in general (training is coming) but I would like to solve the following problem:

I have configured a virtual domain on the server
: "www’.‘mydomain’.‘com’ ", just to host the Wordpress website.
And the mail server is Office 365.

if I use the php mail function or the smtp server of the virtual domain, all emails go well except emails to accounts in the “mydomain.com” domain (eg contact’@'mydomaine.com)
As if the server isn’t sending them (or keep them). And I haven’t configured user “contact” in virtualmin.

Where can I find help to configure correctly and let the mail go? I did not find it in the help …
Thank you very much for your help.

mail server Postfix(?)
Operating system CentOS Linux 8.2.2004
Webmin version 1.962
Usermin version 1.812
Virtualmin version 6.13 Pro