I have an Amazon EC2 Linux box (it's an Amazon Linux AMI basically a Centos) I have a bunch wordpress sites on them, most of them have contact web form. I believe they are using sendmail.
I had an issue, several email sent to a specific Italian ISP got rejected because my server was not reliable, I started googling and I found out that I had to take some extra action:
add something to
change the host name to reflect mydomain.it
That solved the issue, email started to work even with Aruba ISP (the ISP that wasn't receiving email from my server).
Now the problem is with my company website out mailboxes are on Gmail for Business (not on our server) and they look like @mydomain.it...as you can imagine if I send an email from my company website contact form (which is hosted on my server) I don't receive the email, because my server think is convinced to be mydomain.it....
I can't find the guide that I've followed to fix the issue, but as soon as I find the link I'll post it here...