My mailbox is full

Hello Johnnym4,

Thanks for reaching out to us in Apple Support Communities about not being able to send mail due to mailbox being full. We're happy to help by looking into this more! 

There are a couple of options, you can reduce the size of your mailbox by deleting mail or if available you can increase the size of your mailbox. The following support article provides some troubleshooting steps to try for this behavior if you mailbox is an iCloud account:

Mailbox size and message sending limits in iCloud

If you have a third-party email provider, you will want to reach out to them directly for options.

Hope this helps.

Best Regards.