Debian 10.4 / Exim 4.92 add incremental delay to the unsuccessful login attempt answer

Browsed over all the internet, but cannot find an answer – is there any acl or something that I could apply to unsuccessful login attempt to add delay to it? As probably anyone running mx, I have a lot of bots logins trying to guess users passwords. To make dictionary attacks at least a bit more difficult, I’d like to add some (incremental is better) delay after each unsuccessful login attempt’s answer.
But cannot find how to do that with standard debian exim installation. Please share one with a brief explanation. Thank you.

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Author: BbIKTOP