The Mail app works as an email client on Apple devices, and you can add your email accounts to it. When you try to send an important email from the Mail app, at times, the app may fail to Apple mail cannot connect to server. Go ahead to know more about the problems causing the error and the ways to fix it.
1. Remove and re-add the account
2. Check the internet service
3. Restart the device
4. Move the emails to another folder