Hello Stephen,

Yes this is what I am talking about the slow down message that
receiver can send back to the server according to 3 facts :

- the number of SMTP connections at same time
- the number of e-mail within an amount of time
- the number of recipients per single mail

some other protections could also be :

- IP source black list
- SPF DNS check
- greylist system

All  those  protections  can  produce 5.x.x or 4.x.x depending how the
server is configured.

5.x.x errors are "permanent" meaning that the mail has definitely be
rejected/canceled/refused.

4.x.x errors are "temporarily" meaning the mail has also been rejected
but the sender can send the mail again "later".

What you are howing me is a 550 error meaning that this mail cannot be
marked  as  SPAM  as  it  has  been  fully  rejected  by  the server's
destination  so  it  will never reach an inbox with some "spam" tag.

At  the  beginning  you  told about e-mails marked as spam (so meaning
they  has  been  accepted)  and  now you show mail that is permanently
(5.x.x)  rejected  because  they are considered as a spam, that make a
difference.

Also  you  error  message  is  wrong  because  a 5.x.x should never be
retried  !  so  this  "Will  keep  trying until message is 2 days old"
against  a  550  error  number  is  non-sense the server that send the
e-mail should never retry/put in resend queue upon 5.x.x error.

regards.

SD> I'm told there are also limits to the number of emails we can
SD> send to someone per hour, and a bunch of other flavours of anti-spam.
SD> I am seeing mail returns that say:
SD> " message still undelivered after 4 hours Will keep trying until message is 
2 days old"
SD> And
SD> " permanent fatal errors "
SD>  ... " <<< 550-"JunkMail rejected "
SD>  ... " <<< 550 unattended mailbox "


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