I would probably take the easy road and try a simple sort first and check how many times a given domain occurs in your proposed concurrency frame (probably 50-100). Domains occuring more than 2-3 times within a frame may be a problem as your MTA will be asking for that many concurrent connections.

Exim can take care of this kind of things. For instance it looks for all the pending mails going to the same domain and sends then in an unique connection. Read the manual!

In reply to Re^4: Applying the brakes by salva
in thread Applying the brakes by Ryszard

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