[Exim] Splitting mail across servers

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Hello,

I am after some advise. What is one to do when they have to many users on a single server (using one domain) and would want to split mail across multiple servers? What do the larger ISP's do?

I've been thinking about this and believe the best way to do it would be based on the first character in the local-part of the e-mail address/username

i.e. mail to a*@??? to j*@??? goes to server 01 k*@??? to z*@domain .com goes to server 01.

To do this I presume you would need at least three servers 1 being a forward machine and the other two being physical mail hosts.

Obviously I would need a POP3 splitter of some sort, any recommendation would be good.
Then I would need exim to be able to deliver mail based on $local_part somehow? This is the tricky bit I see.

Any idea's/suggestions are welcome.

Thanks

-Scott
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