How to Rate Limit incoming SMTP connection on iSeries?

25 pts.
Tags:
AS/400
iSeries
SMTP
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)?

Software/Hardware used:
ibm i series V 6.1

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Well it seems this can be done via the i Navigator, under IP Polices , QOS rate limits. Trouble is the lowest setting is 1 connection per second. I was hoping to get it a lot lower than this  – anybody have any idea on how to set this to anything other than the minimum allowed by the i Navigator Wizard?

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  • TheRealRaven
    First, such control shouldn't be done on the mail server itself; it should be done at a firewall point. Second, is there a reason you want to limit QTSMTPSRVP only to 20 instances? That's a fairly small number. What is the system environment? What model server? How much memory? How much available DASD? What many active jobs on an average day? Third, is it truly necessary to allow external SMTP access to this server? Could a smaller, less expensive Linux system handle incoming SMTP between the Internet and this server? Fourth, do you have an e-mail administrator with experience? If not, then you should strongly think through the implications of SMTP Internet access. It's not a trivial subject.
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