Unable to migrate a Plesk subscription: Cannot set postbox: Maximum number of mailboxes was reached

Symptoms

One or multiple subscriptions fail to be migrated via Plesk Migratior; the following error is shown:

PLESK_ERROR: Unable to create mail account 'jdoe@example.com' in subscription 'example.com': Command LANG=en-US.UTF-8 ALLOW_WEAK_PASSWORDS=1 PLESK_RESTORE_MODE=1 PSA_PASSWORD=***hidden*** mail --create jdoe@example.com -mailbox true -passwd ***hidden*** -cp-access true -ignore-nonexistent-options failed with exit code 1: stdout:
srderr: An error occured dirung mail account creation: Cannot set postbox: Maximum number of mailboxes was reached.

Cause

The target subscription 'example.com' has a mailbox limit reached.

Resolution

Increase the mailbox limit count for affected subscriptions on the target server as per the following article: How to change the mailboxes number limit of a subscription in Plesk?