-> Jade B:
We had the same issue with iPhones on an earlier build. Our solution involved removing the account in question from the iDevice and re-adding it. The IMAP issues were caused by a BAD iOS Mail app update - and a subsequent update resolved that issue.
We had an issue with folders not populating on iDevices and found that turning off "Threaded Mode" and "Complete Threads" then adding the account caused devices to also download content in sub folders.
We do not use EAS.
On a sidenote - you might want to start a different topic with your issues because they are related to a different version of SM than the one noted in this topic title. Officially, SM's response to your issues will be that you need to upgrade your installation to current - then they will begin assisting with your issues.
MailEnable survivor / convert --