Sent emails automatically deleted

Email in iCloud:

Settings, Your name, iCloud, Turn off mail

Mail in another email account

Settings, Mail, Accounts

Find the account with mail turned on. Delete the account  

If asked if you want to keep them, choose “delete from my iphone ”

Power the phone off and back on.

Then

email in iCloud:

Settings, Your name, iCloud, Mail. Turn it On

mail in another email account

Settings, Mail, Accounts

Add the account back

Open Mail app and all folders should be re-populated.

No Gmail doesn't have 'days to sync' setting, being only pseudo-IMAP

Check the bin/trash/deleted folder and move them into Sent folder. or being Gmail look in the All Mail folder (Gmail has so many places to hide mail)

Same advice regarding trash folders.

Inbox and Sent are system folders and it is not good practice to leave emails in them permanently. They should be moved to user created folders according to subject each day.

I dont know who BT is using now (was Yahoo) but I wouldnt trust them with anything valuable. They should have a forum.

An update cannot delete all your sent mails.

Are they all older than the days to sync setting?

Are they there if you sign into your mail in a browser like Safari?

You're welcome, I hope you find them!