Hi Omar_A1
Many thanks for your response.
I already checked all those suggestions. I am not experience any connection problems, and there have been no service outages to explain this problem. I am only using one email account; BT Yahoo mail, and the mail settings on my Apple devices have not been changed. This problem is completely random; I can send two emails within moments of each other, and one will send and one will not. My iPad makes the Whoosh sound every time, and I do not receive any alerts or messages to indicate that the emails have not been sent. No Outbox appears on my iPad, so I have no reason to believe the emails have not been sent. My Drafts folder is empty, and only when I check my Sent folder do I realise it does not contain the failed emails, which have simply disappeared completely.
I have noticed the Undo Send button occasionally but I assumed it is is new feature in the latest iOS version until I saw the item in your reply. However, if an email has not been sent, why would anyone need an Undo Send button?
I am using an iPad Pro 12.9" 2nd Generation with iPadOS Version 16.2 (20C65), and I have never experienced any problems sending emails until now. I do not make backups. This problem only occurred in the last couple of weeks, and since the last OS update.
Many thanks
JohnnyD049