To start with you can't send mail from @Gmail adrresses from another mailserver then the Gmail ones. In fact any mail you send from any given domain needs to be send from a mail server belonging to that domain.
Since you're trying to send mail from a gmail.com address from your own server a receiving mailserver checks with Google if your domain is allowed to send mail on behalf of gmail.com. Since youir domain is not allowed to do that the receiving mail server will reject your mail instantly. That's why you also don't see that mail in a spam folder, it never reaches it.
I suggest you start reading about SPF, DKIM and DMARC.
If you want to use Gmail you need to send your mail to a Gmail server and have it send to the final recipients from there. You need to configure your Acymailing to use Google smtp. I'm not sure if you can do that with consumer accounts you're using, you might end up with sending limitations.
The best way would be to use Google Workspace and use smtp relay.
Alternativly you can setup email addresses that belong to your actual domain. Please note that the FROM and REPLY-TO need to use the same domain. Then forward any incoming mail to these mailbvoxes to your Gmail accounts.