Yes, but it was about a letter that had been RTS by MyUS, causing Amex to contact me. I chose to ignore it and heard no more.
Most of my USPS-delivered letters from Amex have been accepted at MyUS but there’s always a risk of RTS with ordinary mail, as it’s meant to be “parcels/ packages” unless you have the “mail” option (which is no longer available to new customers I gather)
Courier-delivered mail to MyUS is not an issue, including from Amex. It’s not that Amex can’t “send” mail, it’s that it may not be accepted at MyUS, and therefore RTS (and possibly drawing unwanted attention)