We have an SMTP server that gets into trouble quite often. Incoming SMTP connections (usual suspects are Microsoft mail servers) are usually multiple connections from the same foreign SMTP server (usually Microsoft Office 365 servers). Trouble is for every SMTP connection a new job (QTSMTPSRVP) is created, with a max of 20 jobs. When foreign SMTP servers run out of available jobs they keep opening new connections. It's not unheard of to see 20 to 30 connections from the same IP range (again Microsoft mostly). This just adds to the problem and so the queue gets longer until eventually email clients start timing out. Is there a way of limiting TCP connections from IPs or IP ranges to a certain amount per minute (as in Linux port rate limiting)?
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