On-prem SMTP Relay server slow

[Maxis]

Hi, i have 2 on-prem SMTP Relay server in windows 2016;  the flow is application > SMTP Relay > DLP Server > Recipient;   but starting from last Thursday, i notice all the email always got delay for 2 hours stuck at SMTP relaye server which i can't figure out why.   Everything is good and intact from OS layer,  any advice?   

Both server KB, build, lastreboot, able to tlenet at port 25, able to lookup/ping DLP server, etc all same.  Thanks

[#2]

@Maxis 

Any change and update recently including DLP policy?

[#3]

@Kidd_Ip   i did check with security team, no change on their end;  funny part is node 01 no delay;  02 delay for 2 hours if do the message analyst.  but can't figure out the root cause.