‘550 no rDNS entry for xxx.xxx.xxx.xxx’ means that there is no reverse DNS record for the sending email server.
You may see this error in logs :
Tue 2018-09-04 10:36:46: <-- 550-Sending server has no reverse DNS (PTR record)
Or you may get a bounce back message :
firstname.lastname@example.org>: host mail.[destination_ip] said: 550 No rDNS entry for 100.200.300.400 (in reply to RCPT TO command)