Sometimes your mailbox created in the Plesk Panel reach the default outgoing message limit. and you will require to increase the message limit. Following are the steps to increase the outgoing message limit in Plesk.
- Login to your Plesk Admin Panel.
- Click on Tools and Settings.
- Inside the Mail section click on Mail Server Settings.
- At Settings Tab, you will find the option "Default limit on outgoing messages from a mailbox (per hour)" change the limit as per your requirement and click on Ok Button.
Now, you should be able to send emails without any issues.