Help we cannot send emails, what to do?

hello, we are getting notifications when sending email since two days, before we had the problem sometimes, but it solved itself after waiting some moments.
the notifiaction we get is:
[SMTP] Kan het bericht (‘Van’ met het veldadres ‘’ dat niet is geaccepteerd om de volgende reden(en):
"5.5.0 SMTP protocol violation: A mail transaction is already in progress. See RFC 5321, section and 4.1.4
") niet verzenden.
i get the same message with my mailaccount
I checked the IMAP(= port 143)and SMTP( port = 25)
we have our emails from em client, we are using windows 7, we checked: we had no update taken place, our internetprovider is t MOBILE.
What can i do now?
plese Help!

I can add: this is how we have managed the authification: