AGPL3 shouldn't be a blocker for use with this sort of tool unless:

▪ someone is very paranoid about GPL infection (that is to say that they, or their bosses, have been taken in by some of the fear-mongering over the years)

▪ or they are intending to make the feature available as part of the their product/service (if it is a mail related/adjacent tool and they want to use this as a built-in self-test module) rather than just using it internally, in which case they might be subject to the full terms of the licence due to effectively directly linking the code.

To alay the concerns of that first group, perhaps include in you documentation a paragraph explaining that simply using it in a dev environment, with no redistribution, does not constitute linking.

If someone tells you "no one will use it commercially if you use GPL"¹, you always have the option (assuming all the code is yoursor contributors have signed over their relevant rights) of dual licencing GPL and commercial.

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[1] this usually means "I want to sell this with my service but don't want to pay or otherwise give back, please use a more permissive license so I can do that"