Too many SMTP Auth Failure

Hi.

I can see in the log so many rejected mails because of SMTP Auth Failure. While most of them are coming from strange email addresses, I'm worrying about many that are coming from our domain!!!!

Fortimail is doing a good job by rejecting these emails, as they are from unknown Client/IPs, which means that someone is trying to send emails from my domain, but he's not actually my real user.

is it a good practice to keep the current setting and continue seeing these rejections? or there is a better configurations specifically for these kinds of issues?

Not sure if this is related to impersonation or authentication reputation or somewhere else, but I really feel that I must prevent these emails in a very solid way.