We received email "Undelivered Mail Returned to Sender"
Within these few days, when we sent email to guest with "@guest.booking.com", we received email (there are many of these emails) below just one example:
from: Mail Delivery System <***> to: [our email removed] date: Mar 7, 2021, 11:08 PM subject: [Booking.com] Undelivered Mail Returned to Sender mailed-by: ***.booking.com
This is the mail system at host mailrouter-208.lhr4.prod.booking.com.
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can delete your own text from the attached returned message.
The mail system
<our guested email removed>: host intercommail.booking.com[10.252.13.123] said: 530 5.7.0 Must issue a STARTTLS command first (in reply to MAIL FROM command)