I know this has been touch on before, but we sent out around 4,500 emails using CiviMail via Sparkpost. 21% were bounced!! and most of them have the bounce type of "Syntax". Here is some examples below.. https://snag.gy/5ARxyc.jpg
authentication failure [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at Administer CiviCRM >> Global Settings >> Outbound Ema
Can anyone help me with this error, as there is no real error code / message.
Many thanks in advanced.