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Synology MailPlus email auto-forwarding

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I am running MailPlus server on the daily basis. However, I noticed that there is an issue with an email auto-forwarding. Here is my test:

If I receive an email from Gmail on myinbox1@mydomain.com, I can set up an auto-forward to myinbox2@mydomain.com. It works as expected. However, If I set up the auto-forward email to a mailbox on Outlook.com, then it failed. From the DSM MailPlus "Audit" windows, I can see the error saying "Undelivered Mail Returned to Sender." The sending queue is empty.

I am using Dynu SMTP relay service to send emails. It works great if I write an email and send it or manually forward it through my MailPlus client.

Did I miss any settings? Please advice.

Update: Confirmed it is not MailPlus issue. It is something to do with the relay server. I changed Dynu relay server to my ISP's, the auto-forwarding works as expected.

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