Why: Couldn’t add messages to library (MCMailErrorDomain error 1030.) – Mac Mail
Did you try to delete or move email messages in Mac Mail and receive an error message?
The operation couldn’t be completed.
(MCMailErrorDomain error 1030.)
Couldn’t add messages to library
If you attempt to delete or move the email messages again, they likely succeed. It appears to be an issue with the Mac Mail email account that possibly experienced a connection issue briefly (such as GMAIL or YAHOO).
If moving or deleting your email messages a 2nd time does not succeed, try to quit and re-open Mac Mail and performing your action again.