When you have a contact that suddendly experience ERROR AFTER DELIVERY what does it mean ?
We call this : BOUNCEBACKS. The most logical explanation here is that the Contact's address is no more valid.
Destination servers typically return "text format" emails (for example, generated by Exchange). Thos messages say : "Sorry, we finally couldn't deliver this message to the recipient".
Those types of messages are handwritten by the server administrator in his own language and they often put a full copy of the message, so the size can be more than 200Kb so Ofsys doesn't keep a copy of it, Ofsys just keeps the fact that a Message was returned (Bounceback). In summary, the answer is not a clear error code and not a standard SMTP message.
In certain very rare cases, these messages arrive back to Ofsys with a clear "DSN status code" like (ex: 5.1.1) and in this case we also treat them as quarantines.
IMPORTANT : Other ESPs (Email Services PRoviders = Email marketing platforms), hide or ignore those errors because they are not easy to treat. At Ofsys we made the choice to show them.
We made the choice to show these kinds of errors because we want our clients to see what's happening and discover that something's strange is happening. This will allow our clients to make better decisions like "it's been 1 month that this address doesn't work, so let's de-activate the contact".