Update: The issue seems to have been resolved and Laposte say they’re no longer sending the 519 responses as of April 25th 2018.
Laposte.net are having a bad couple of weeks. There’ve been reports from customers of their IMAP service being unusable, with attempts to move or delete messages timing out and expected emails simply not arriving.
Several delivery friends have mentioned that they’re rejecting mail with errors that look like this:
550 5.5.0 Service refuse. Veuillez essayer plus tard. Service refused, please try later. LPN007_519
It looks like they have some serious mailstore problems. They’ve said that the “519” bounce code means your mail won’t be delivered, but it’s a problem at their end and there’s nothing senders need to do to resolve it.
Their rejecting mail with a 550 response (“Requested action not taken: mailbox unavailable”), rather than a 4xx response that would allow for retries is probably a good operational decision for them if their infrastructure is melting down, but it does mean that recipients at laposte.net are likely to bounce off lists. You should check on that, and add the bouncing addresses back.
Laposte’s statement to their customers says (badly translated from the French):
Alert: delay in receiving your mails
The situation has improved in recent days even if it is still unstable for some of you. This unprecedented incident is notably due to an exceptional growth in the number of advertising mails (multiplied by 10 in recent months).
We mobilized all technical expertise to restore optimal service by installing new servers. We carry out safety and scalability tests to ensure a lasting recovery of the device.