Hello @GuillermoO ,
It's possible that the particular contact's security setup, or how they've taught their email program's algorithm to treat certain email elements (whether consciously or otherwise) has affected your ability to deliver to them. Email programs' algorithms - especially in regards to spam filtering, bouncing, and warning messages - can change depending on a myriad of circumstances.
Your account lacks email self-authentication despite having its own unique domain, which can lead to delivery issues - including your emails being blocked from receiving. I'd recommend setting up self-authentication for your account, and to contact our Delivery team if you need further insight or guidance in either of these regards.
... View more
