When trying to delete a message in Mac Mail you receive an error message similar to the following:

"The IMAP command “UID COPY” (to Deleted Messages) failed for the mailbox “INBOX” with server error: Failed to save copy of message.."

This is a known bug in Mac Mail. 

To resolve this please try the three options below. 

-After trying the first one save the settings, close mail, reopen then try to delete a message. 

-If the error persists please try the second option close out of mail, reopen, and try deleting a message once more. 

-If the problem still persists try the third option close out of mail, reopen, and try deleting a message again.

Option 1) Go to your > Mail Preferences, > Accounts (choose intertune mail account) > Mailbox Behaviors - Untick Move deleted messages to the Trash mailbox.  (this option may not be present in newer versions of mac mail 10, 11 proceed to option 2 or 3)

Option 2) Go to your > Mail Preferences, > Accounts (choose intertune mail account) > Mailbox Behaviors - then under Trash Mailbox, select On My Mac Trash bin.

Option 3) Go to your > Mail Preferences, > Accounts (choose intertune mail account) > Mailbox Behaviors - then under Trash Mailbox, select NONE.