Mail won't open. It is blank.

Try the Rebuild command from the Mailbox menu.

The list of emails is generated from a database that is created from the raw messages. If the link between the index and the raw message is broken, it can't display the message.

The Rebuild command rebuilds the index.

Maybe read this then try posting again. You didn't even tell us what system. Is this in webmail?

Writing an effective Apple Support Commun… - Apple Community