Fortigate 100E can't allow certain email to go through (SMTP)

[JackyLim]

Hello everyone,

Our company uses a Fortigate 100E, previously we had a email junk spammer so we decided to tag our emails. Slowly even non-junk emails got tagged and it became a huge problem so we turned the tagging system to discarded for SMTP. However, a week later we got attacked by RDPs from multiple IPs and we decided to ban them manually, which amounted to 150+ to date.

Now the problem we face is some big companies can't send emails to us because it somehow gets rejected (not logged in our Exchange or Fortigate) or it takes about 8-9 hours to receive them. It could be that they are using Office 365 and we are using an old Exchange 2010 server, I'm not too sure if that could be the issue because before we touch anything in Fortigate everything was working smoothly. My guesses are there are many RDPs that causes heavy traffic hence the delayed emails or we might have disable a feature and there was a hidden feature we did not turn back on.

Not too sure if this is the correct section to my issue but I do apologize if it isn't. We would appreciate any solution.

Jacky

[JackyLim]

Going to answer my own question. The emails suddenly routed to another existing IP under our domain. That IP mailbox server is inactive. Our main IP was somehow blacklisted suddenly and only Microsoft decided to not use our main IP and switched to the inactive IP. Gmail and Yahoo and others could send us mails only Microsoft related services won't work (exchange, hotmail...etc.etc...). Quick Update: Fortigate 100E had blocked a lot of RDP attacks but it seemed like some malicious items have passed through the firewall and got our main IP blacklisted.