I can't reply to emails that are arriving through a Wordpress contact form to one of my email accounts. Yesterday and the days before it worked well. Now I'm getting Undelivered Mail Responses with error 550 which is nonsense as I mananged to send emails to the recipients from my private gmail account.

I have attached an example of the email that arrived and I tried to reply to and another one about the error message.

The other weird thing is that these emails that are coming from my Wordpress contact form also end up right away in my Sent folder, no clue why...

Please fix this issues ASAP as due to Holidays are close I can't properly communicate with my clients.