Sent emails missing from my iPhone that I sent one day ago

Suddenly all my sent emails have disappeared except for one I sent two days ago.

I have tried toggling iCloud on and off and turning phone completely off, some people recommend a setting about syncing but there is no setting for that as other people have said too. I have bigpond email and an IPhone 8 Plus.

I’ve tried changing setting to “save to server” and save to phone.

I have tried logging on to Telstra webmail but it’s the same.

I’m too scared to delete any accounts when I don’t know what I’m doing.

can someone help please get my sent items back as there are some important documents.

[Re-Titled by Moderator]

Original Title: Sent emails disappeared