Then please do name "the mail-based virus checkers that kill the checking process if it takes too much time/memory and not deliver the mail".

I spent several years in a company that developed this sort of products and I am not aware of any commercial products that do what you described. I am guessing you are referring to the mail gateways integrated with FOSS virus scanners, and still I doubt the behavior you described is typical even between them. From what I saw the restrictions were implemented in the scanners themselves, and the mail that could not be readily processed was always quarantined, and not simply "not delivered".