I have Virtualmin setup on my VPS with Ubuntu 18.04. I have one virtual server on the system and I added an email user in that domain
I'm using Thunderbird as my email client to send and check mail. This is configured to receive POP3 on port 995 and send with SMTP on port 465.
Using Thunderbird I can send email from the account I created to both that same account and also a different (gmail) account and they are received without a problem.
But when I send replies from those emails, the reply from the same account arrives without problem but the reply from the gmail account hasn't been received. Why does one work but the other doesn't?