SMTP
334 Server Challenge
The server is issuing an authentication challenge as part of the SASL authentication exchange. The response contains a Base64-encoded challenge that the client must decode and respond to.
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During the AUTH command handshake. The server is prompting you for your username or password (Base64-encoded) as part of the multi-step authentication process.
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Respond with the appropriate Base64-encoded credentials. If authentication keeps failing after responding, verify your credentials and ensure you are using the correct SASL mechanism.