My BT Internet emails disappear from my iPhone 8 inbox after about a week. I've no idea where they go as they're not in Deleted, Bin, or Archive folders on the phone. No problems on my iMac which shows all emails in Mac Mail inbox.
This does not happen with my Gmail email account on the phone.
I've tried deleting the BT Internet mail account and re-installing it on the iPhone, but made no difference.
Solved! Go to Solution.
If the mail is present on other devices and webmail,it will be a setting in your mail app determining how long to sync mails.
Fixed the problem; I freed up more storage on the iPhone which seems to have kept emails longer in the inbox. So nothing to do with email settings!