I've hacked sendmail to make an ioctl call at the time of the SMTP RCPT command, to check if a user has exceeded their email quota. If they have I issue a temporary failure and a message - can I do this with Exim?

If you can make this happen in Perl you can use the embedded Perl facility, and use it from a condition condition in an ACL statement. You can also use the expansion facility to run an external program, but this uses more resources because it uses another process.

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