Campaign mails not sending

[imn]

Hi,

I have set up an email campaign within Sugarcrm 5.2j.
Many times i have send a mass mail to all my contacts. With succes.
I did this with an exchange mailing account.

But for some reason, last night, i kept getting the error message
All the test mails were succesfull.

FATAL] SugarPHPMailer encountered an error: SMTP Error: Could not authenticate.
FATAL] SugarPHPMailer encountered an error: SMTP Error: The following recipients

It also takes a very long time before a single mail is send.

When i change my settings to gmail it works perfect. My test mails are being send at a very fast manner.
But that's not what i want, because the reciever will get a mail from a gmail account.
Thats not acceptable.

Can somybody help me with this?