Keep mailboxes from getting too big in Mail on Mac
Here are suggestions to try if mailboxes are so big you can’t find messages, or you want to reduce the amount of storage space that Mail uses on your Mac.
In the Mail app on your Mac, do any of the following:
Delete messages you don’t need anymore, especially those with large attachments.
Save attachments to your Mac, then delete the attachments from messages.
Create more mailboxes to organise your messages. For example, if you have messages related to travel plans, you could create a mailbox called Travel and move travel-related messages to that mailbox.
Use rules to automatically move incoming messages from your Inbox to other mailboxes. For example, you could create a rule to move travel-related messages to the Travel mailbox as they arrive. You can also use rules to process messages already in mailboxes.