I recently switched my mom to use IMAP instead of POP on her devices. She has an iPhone, iPad and a Windows PC. Moved all local mail to the Comcast servers and recreated them on each device. They are all setup as IMAP. Outlook on the PC is fine. It downloads copies of all the emails on the server including like 11k of sent messages. The iPhone and iPad download "stuff" however the sent log is kind of odd.
In the send log it will download say 3k of messages which seems to be some kind of limit. Seems to download mostly oldest first (2010 on) and a few from the last couple days and one from 2016. Then later I can go into the Send area and nothing happens. Like no additional downloading of sent messages. I start scrolling down the list of sent messages and eventually it will start downloading again. Not all of them but another group. Two things can happen at this point. Either the added messages will stick if I follow a process of "go out of sent to the inbox, close the app, launch the app and go back to sent". OR, doing that it will start refreshing and all those messages just downloaded will start going away as I watch it and be left with the messages initially downloaded. Over time it will download a bit more but not sure what's happening here.
I've even seen where messages from 2023 show up at some point and I can click on them but again go out and in again and they visually start disappearing from the sent folder.