Hi Com007,
Thank you for posting to Apple Support Communities.
To help fix issues with your mail account on your iPhone, you can remove the account and add it back again. This allows the account to reconnect to the mail servers and can resolve this. Here's how to do this:
1. On your computer, sign in to your email provider’s website. Make sure that all of your email is there or make sure that your email is saved somewhere other than on your iOS or iPadOS device.
2. On your device, go to Settings > Mail, then tap Accounts.
3. Tap the email account that you want to remove.
4. Tap Delete Account.
5. Add your account again.
This information is provided in If you can’t send email on your iPhone or iPad - Apple Support
Take care.