VPOP3 (a feature of QMail) never generates an SMTP error 554. If you are getting an error message from VPOP3 about an error 554, it is because your ISP is reporting the error to VPOP3 and VPOP3 is just passing the message back to you.

The message text following the 554 error (or in the Message Delivery Report in the bounce message) may give more information about the meaning of the error, or you may need to contact your ISP for more details.

All the SMTP Standard (RFC 2821) says about error 554 is:

554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")

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