Lookup for "214 4.2.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (214)
Help message [Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user]
Enhanced Status Code Details (4.2.4)
Mailing list expansion problem
The mailbox is a mailing list address and the mailing list was unable to be expanded. This code may represent a permanent failure or a persistent transient failure.