I've also just updated to iOS 13.2.3, and I am using the Apple mail app with an outlook.com account. The "double delete" issue (where if you delete a newly arrived mail, especially if you delete it quickly before it has synced to read status, meant the screen flashed and seemed to delete it, but you would then be shown the same mail again, having to delete it a second time before it actually was deleted) seems resolved for me, as others have also mentioned. This fix was not mentioned explicitly in the 13.2.3 change log (unless it is related to fetching of new messages which affected some people and which is mentioned).
I also suffered from forwarded messages losing all the quoted content at the recipient end. This is explicitly mentioned as fixed, and I haven't been able to reproduce it either since upgrading, so fingers crossed (though it was intermittent previously, based I think in part on the formatting or length of the quoted message).
However, the most obvious issue - duplicates in Sent Items for sent messages - remains! For me at least, and many others, this is not an intermittent glitch, but happens for every sent mail - again, depending on the formatting or content of the message, sometimes one of the duplicates disappears when opened, sometimes not. Clearly a bug in the Apple mail app with Exchange accounts. Apple, please fix!