SMTP Error Code: 534 4.7.2

what's your mail server saying?

Lookup for "534 4.7.2"

Temporary Error - This status code indicates a transient issue and may resolve itself.

Basic Status Code Details (534)

Authentication mechanism is too weak

Enhanced Status Code Details (4.7.2)

Mailing list expansion prohibited

The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error.