Lookup for "535 4.2.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (535)
Authentication failed due to invalid or insufficient authentication credentials
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.