[Exim] Variable max message size

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Hi

We wish to be able to specify different max message sizes on a per user
basis, for both incoming and outgoing mail.

We are just going to store the sizes in a plain file using the format

local_part max_message_size

Which part of the exim configuration is the best place to control this
type of thing? And how would i get it to search the file and check? Im
guessing id use lsearch but im not sure.

thanks
Simon
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