Gnus does handle email in a potentially unusual way by default, but it's not too difficult to change that. For example, you can change it to mark articles as dormant, rather than read by default, when you read them so that they won't be subject to expiry. This presumes that you're using total-expire for the relevant group, which means that read email expires after N days. Given that alteration, then you'll just explicitly hit 'd' to mark an article as read, which will schedule it for expiry. Gnus' normal delay for deleting emails can be handy sometimes, but you can always force an immediate delete if you need to.
I think you just proved that GNUS is not a good choice for most people.
I understand that the concept of expiring comes from reading usenet postings, but it definately shouldn't be applied to email, at least not by default. I've used GNUS before, even for reading email, and it really can be flexible and capable. But it's much more confusing than it needs to be.
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