Is it possible to move multiple messages from one Mbox to another?
Is there a maximum or optimum number of messages in an Mbox?
Yes, you can drag the messages to a folder (which will convert them to .eml files) and then select those files and the destination mailbox and choose Record ‣ Merge Message Files.
No, it really depends on your access patterns.
@missedbeat Note that this doesn’t modify the source mailbox but hides the messages in EagleFiler’s view of it. If you feel the need to excise the messages, see Deleting Messages in Mailboxes for a procedure.
Thank you both.