contoso-com.mail.protection.outlook.com[xxx.xxx.xxx]said:451.4.7.500 server busy. please try again

we have users that sends email to O365 user mailbox, mail sending failed with error below

host contoso-com.mail.protection.outlook.com[xxx.xxx.xxx]said:451.4.7.500 server busy. please try again later from[xxx.xxx.xxx.xxx] (s777230)[HK2APC01FT138.EOP-APC05.PROD.PROTECTION.COM

any known fix?