Postfix reports deferred status for outgoing mails 451 4.7.500 Server busy

Issue

The mailq shows that mails are stuck in queue with following error.

7D3BF2337DD9    42510 Mon Jun 13 16:47:30  from-username@domain.com   (host relay-server.mail.protection.outlook.com[x.x.x.x] said: 451 4.7.500 Server busy. Please try again later from [x1.x1.x1.x1]. (S77714) [sender-name.protection.outlook.com] (in reply to end of DATA command))                                        to-username@domain.com

The /var/log/maillog reports similar error

Jun 13 15:27:57 hostname postfix/smtp[84101]: DB32123439EE: to=<to-username@domain.com>, relay=relay-server.mail.protection.outlook.com[x.x.x.x]:25, delay=2.8, delays=0.63/0/1.6/0.56, dsn=4.7.500, status=deferred (host relay-server.mail.protection.outlook.com[x.x.x.x] said: 451 4.7.500 Server busy. Please try again later from [x1.x1.x1.x1]. (S77719) [sender.protection.outlook.com] (in reply to end of DATA command))