Hi KieranB90,
Since this also happen with webmail, it sounds like the issue is related to the email account itself, not your device. In that case, the email provider can assist further. If you use a third party email provider, reach out to them directly. If you're referring to emails that you're sending from iCloud Mail, check to make sure that your sent items are being sent to the correct mailbox. Try the following on your iPhone.
Go to Settings, then tap on your Apple ID at the top of the page. Tap iCloud > iCloud Mail > iCloud Mail Settings. Go to Advanced and then Sent Mailbox. If that says anything other than "Sent Messages" tap on it and change it. Then try again.
Take care.