My wife has an IMAP account (mine is POP, so this question does not apply to me). She deletes as many emails as she can but still wants to keep many of them without going over the storage limit of the server (and she can come close!). If I set up a dedicated folder in the On My Mac section of Mail, can she move emails to that folder and have them stored on our computer and delete them from the server at the same time?

There are multiple options for transfering a message from an IMAP mailbox to a mailbox "On My Mac". But whether she drags and drops, Move to…, or Copy To… the copy in the ''On My Mac" mailbox is disconnected from the copy in the IMAP mailbox. Drag & Drop or Move To… automatically deletes the copy in the IMAP mailbox, Copy To… results in an independent copy in both mailboxes.

NOTES:

• the space on the IMAP server may not be immediately freed up by a Move operation depending on the specific host server settings so she would still need to be diligent in choosing and moving messages from the server.
• if your wife keeps and frequently references a lot of email, she might want to consider downloading them into a database such as Yojimbo, or DEVONThink 3 that would offer multiple ways to organize and search messages; or a dedicated email archive utility like Eagle Filer, Email Archiver Pro, or Mail Archiver X Easy that would link directly to the IMAP mailboxes and store the messages locally.
• The Pros and cons of each type of utility could be a subject in and of itself, if she — and you — want to explore this further.

It automatically moves the copy in the IMAP Inbox to the IMAP Trash mailbox. Its complete removal from the server depends on the settings in Mail for that account. (At least in High Sierra, the range of choices for Erase deleted Messages: includes Never, After one day, After one week, After one month, and When quitting Mail.)

Thanks, Joe and DK. I will examine those options to see which is most suitable.

I am under the impression that "Erase deleted messages…" applied ONLY to messages in the Trash mailbox? At least that is how it appears to work in Mac OS 11 and 12.

Sorry to be unclear. I was referring specifically to messages that Jon's wife had already deleted, since he said


I just wanted to clarify that server space continues to be occupied by those deleted messages until they're erased from the trash, which may be some time later.