I would think that email providers tend to not offer E2EE just because it fundamentally isn't practical with email. Providers like Proton try to do it, but it works only if you talk to someone else on Proton (at which point you may as well do it on Signal, which has better encryption).
Back in the day Exchange offered SIS but in 2010 they ditched it. It's plainly not effective any more. Even regarding the OP' "the jpg signature logo" - it's a part of multipart in the message, not a separate file.
And one more thing - you can't just turn the dedup and be dandy, now you need to check against the hashes to determine if this chunk is unique or you already have it. And with TBs of data you need TBs of hashes to check. Until you have like 99% dedup efficiency, ie 99% of your incoming data is literally the same data you already have - it doesn't worth it.
https://techcommunity.microsoft.com/blog/exchange/dude-where...
Or if everything in the year 20XX gets pushed into using E2E encryption, since that's pretty much antithetical to deduplication.