This is usually because your mailbox service thinks the email is spam.
View the bounced email
The email is automatically deleted in 7 days.
Please consider the following options:
  1. If the email is not spam, you can create a filter to explicitly allow all emails from SimpleLogin.
  2. If this email is indeed spam, it means your alias {{ alias.email }} is now in the hands of a spammer. You can either disable the alias or block the sender if they send too many spams.
Please note that the alias can be automatically disabled if too many emails sent to it are bounced.