SMTP Server says "Queued for Delivery", but Email Never Arrives

Would appreciate if you can shed some light on why does the log show that emails are not sent – instead they say “Queued mail for delivery”? According to the User (of my software), the email never goes out. This is happening (suddenly) to ALL their outgoing SMTP emails using Chilkat.

For example:

...
    dataCommand:
      SmtpCmdSent: DATA<CRLF>
      SmtpCmdResp: 354 Start mail input; end with <CRLF>.<CRLF>
    --dataCommand
    mimeDataSize: 234098
    smtpSendData: Elapsed time: 0 millisec
    endOfData:
      SmtpCmdSent: <CRLF>.<CRLF>
      SmtpCmdResp: 250 2.6.0 <x> [InternalId=x, Hostname=x] Queued mail for delivery
    --endOfData
    smtpFinalResponse: Elapsed time: 281 millisec
    smtpConversation: Elapsed time: 281 millisec
    Success.
  --SendEmail
--ChilkatLog