JUSTOX3000 wrote:
An e-mail with a large attachment is stuck in the outbox. Apple Mail hangs (working memory approx. 10GB). Email can only be deleted temporarily. When Apple Mail is restarted, it is back in the outbox.
How can I delete the e-mail permanently?
Look at your Mail>Settings>General
if "automatically try sending later" is checked here. Uncheck the option and try again deleting your stuck outbox.

if no progress, quit the Mail.app and relaunch it holding the Shift key down and try again
10GB is huge for most any mail server to attach....
Even the special iCloud "Mail drop" feature is limited to 5GB
ref: Add attachments to emails in Mail on Mac - Apple Support
ref: Mail Drop limits