recently my server goes down a lot due to high resources usage from xoops. I notice that sendmail use the most resources and I have to do killall -9 sendmail everytime to resume the server's resources. I wonder if there's a way to limit the use of sendmail and why is it using so much resources. How do i unactivate this sendmail process ?
Many thanks

In your mail setup section of XOOPS you can also set XOOPS to use the phpmailer. As for sendmail, there are entire textbooks devoted to the art, so I have no doubt that you could configure sendmail to only accept X amount of mail from XOOPS at any one time.

A little more about the server might help.
Sendmail shouldn't be bringing the server down or even using that much resources even if it was trying to send thousands of messages. You have something outside of XOOPS broken.

Possibly even an open sendmail process hijacked by spammers? Check the sendmail logs.

Probably did, I check the log and there's a huge list of sending emails. How do I stop this or should I install some kind of firewall to stop spamming? I'm pretty sure someone is messing around with it. Server was running fine couple weeks ago. Recently it starts to crash frequently and when I do the "top" command, I see sendmail use up to 50% of CPU and server load can sometimes be as 30.00 high.
Thanks

When i go into the Mail setup section, I receive an error code 500. :(

First thing is to shutdown sendmail right now. Kill all the dangling processes, now. Then analyze your needs, do you even need sendmail running/listening at all?

You should have a firewall but that won't fix your sendmail mis-configuration. You are configured to allow mail relay from any source and the spammers have now found out. Can you say a million messages per day and a very angry ISP?

Does the server recieve email or relay mail for other machines? If not you don't need sendmail running at all, the local machine can spawn sendmail when it needs to. If you do need it running you must configure it to only allow hosts to use it that you trust.

Configure the local access tables to allow only services you need and block the rest. But this gets into things way beyond XOOPS or the scope of this message.