[PHP] Delay in a mail item.

One of your mail items is currently undeliverable at this site, and is on a queue for retransmission. A retransmission attempt will occur periodically for the next 4 days. The usual reason is that the host that we tried to send your mail to is inaccessible via the internet (presumably temporarily)

[PHP] Your mail could not be delivered.

Your mail could not be delivered and is being returned undeliverable. Reason: too many retries - giving up. The smtp log of the transaction was: *** dns_lookup: Trying a lookup on "1.8.78.129.in-addr.arpa" dns_lookup: Fou