BT Not Accepting Emails
This was first noticed at 12:09 on Saturday 9th January 2016. It appears to affect emails sent to emails addresses ending @btinternet.com and @btopenworld.com although we have noticed the same message for @sky.com, @ntlworld.com and @hotmail.com but the latter three messages were only transient whereas the error messages from BT have lasted for several days. The error was last noticed at 11:37 on Monday 11th January 2016.
451 System policy engine error
The error code 451 indicates that this is a transient error and that emails can be retried as they may get through later. After 48 hours, messages to @btinternet.com and @btopenworld.com were still not getting through.
Diagnosis of the error
Emails were being sent to an email address that was not @btinternet.com or @btopenworld.com but rather to a domain owned by the recipient. These emails were being forwarded to the recipient's @btinternet.com or @btopenworld.com email account. Some were being delivered to the recipient's @btinternet.com or @btopenworld.com email account but others were being rejected with the error message 451 System policy engine error.
It would appear that ALL the emails that were being rejected were being sent from email addresses such as @bounces.amazon.co.uk, @eu-west-1.amazonses.com, @bounce.upworthy.mkt5937.com @bounce.email.secretescapes.com and many more!
What these senders all have in common is that they have s SPF DNS record that specifies a default of "-all". This means that emails from those domains are only ever expected to come from specified IP addresses and that receiving email servers should be suspicious of emails from other IP addresses. This gives a real problem for recipients who are using email forwarding!
How Forwarded Emails are Affected
The Journey of a Forwarded Email
When a company such as Amazon sends an email to a recipient it will travel from the Amazon server to the recipient's server. With most recipients, the email will end up in the mailbox for the recipient. Some recipients forward their email to another address. For example, you may have given Amazon your personal email address but you set up your email address to forward any email to your work email address. In this example we will assume that all email is being forwarded to a @btinternet.com email address. So, in this example the emails journey is:
Well, that is what should happen. But...
Why BT Blocks Some Forwarded Email
When a email server receives an email it usually does many checks to see if it is spam. This is vital as more than 90% of emails travelling over the Internet are spam. So, using the above example the emails journey is:
What to Report to BT
Tell BT that you are using email forwarding from your non-BT email addresses. The forwarding will send emails to your @btinternet.com or @btopenworld.com email account. These forwarded emails are very important as, for example, they contain parcel tracking information from Amazon. BT should accept your forwarded emails even if the sender (e.g. Amazon) has an SPF DNS record with a default setting of "-all".
Why Does Not GMAIL Have the Same Problem?
Gmail does not have the problem as it only uses the SPF DNS record as an aid to detecting spam and, unlike BT, does not reject only based on the SPF DNS setting. More details on how Gmail handles forwarded emails can be found at support.google.com/a/answer/175365.