Transient (4xx)
452 4.3.1
Insufficient system storage
The receiving mail server itself is out of storage, unrelated to the recipient's own mailbox quota.
452 4.3.1 Insufficient system resources
What it means in practice
4.3.1 means the receiving mail system itself is out of resources. It's the recipient organisation's disk or memory, not the individual mailbox quota (that's 4.2.2).
On Microsoft Exchange this is the back pressure response: when the transport server runs low on disk space or memory it stops accepting new mail and answers 452 4.3.1 Insufficient system resources until it recovers. Nothing on the sending side fixes it. Let your retries run, and if one domain stays stuck for more than a day, tell their IT team, because their own inbound mail is stalled too.
Common causes
- Receiving mail server's disk/storage is full - a problem on their infrastructure, not yours.
What to do
- →Retry later. Nothing to change on the sending side.