Campaign mail suddenly strips HTML header

[HaKr]

SugarCRM 6.1.0 (Build 5389), Microsoft Exchange server as SMTP server

We send campaign mail on a regular basis. The last one was divided into four related campaigns. Three of them went out without any problems. From the fourth we received several replys stating that the mail was not understood.

From the reply it seems that the HTML tags upto BODY were not included in the mail sent. See the attachments that show that the first lines are missing.

Does anyone know how this could happen? The logs show no indication that something went wrong. After the recipients complaint, I sent out the test e-mail again, resulting in a correct e-mail.

Any tips woul be greatly appreciated