@Chau Le Apologies for delay on this.
The local DNS might have been going through some bandwidth issue, which will cause the mail server to take more time for finding the correct SMTP server address for the receiving end.
Public DNS are normally faster which are backed by huge Infra setup and load balance the request quite well than the local DNS servers.
Normally a RMS protected mail, when received the client will try to fetch the RMS SRV records under DNS to find the RMS servers, the lookup frequency in case of Google public DNS is also better due to their Edge nodes which local DNS/infra will lack.
If you want to investigate more, you can open a support case with us to dig further.
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