[cketti]

I understand it's difficult for a company to give money to an open source project. But none of the options you've mentioned sound very appealing to me.

GitHub displays the avatars of sponsors. Maybe that's an option that satisfies the "acknowledges sponsorship" criterion? https://github.com/sponsors/cketti

[fxtentacle]

They could release a paid upgrade that allows you to customize the splash screen to show your company logo

That's clearly advertisement and paying extra to have your logo on a product is an established business practice.

[Cu3PO42]

You could also sell access to some "extra documentation". It doesn't have to be particularly useful or add any value at all, it just has to be there. You could probably go as "far" as just writing a two-page rundown of IMAP or something. Maybe even put it online for free and sell access to a PDF. The point is there's a tangible "benefit".

This does not apply solely to taxation, but also what is an easy sell to management.

All of this is extra administrative work that understandably you don't want to deal with, but it may open the door to significantly more sponsorships, so it could be worth looking into.

Disclaimer: I am neither a lawyer nor a CPA.