Hi there ZYC-,
It sounds like emails are disappearing as you're writing them on your MacBook Pro and iMac. We're glad you've joined us in Apple Support Communities, and we can provide some guidance.
To start, "For Exchange and IMAP accounts, drafts are stored on the mail server (unless you select a mailbox on your Mac in the Mailbox Behaviors pane in Mail preferences). As soon as Mail syncs with the mail server, your drafts are available wherever you use that account. To sync the drafts immediately, choose Mailbox > Synchronize > [account]." Have a look at the links below to learn more about drafts and composing emails:
Save draft emails in Mail on Mac
Write and send emails in Mail on Mac
Next, are you switching to a different app on your Mac before this happens? If you click and hold the Mail icon in the Dock, and then click 'Show All Windows', are you able to see the email you were composing? The links below provide more information:
Manage windows on Mac
See open windows and spaces in Mission Control on Mac
Take care.