Hi @Deltodeus
Check what happens when you manually send the email to them? Do you get a "delivery failure" autoreply email to your inbox.
Refer the subject of the delivery failure email and see if there is a pattern between that email and the original email.
Just an example:
Original Email:
Subj: This is my original email
Failed Email:
Subj: Undeliverable: This is my original email
If this is the case save the original email subject (maybe in online spreadsheet or sharepoint) and use it for comparison if your flow has to wait for several days for the delivery failure emails.
You will need to create a proper process to also handle things like two original emails with accurately the same subject.
