Possible duplicate headers issue when sending an outbound email with attachment

When sending an internet email with an attachment to certain internet email services, this type of bounce message is returned:

This delivery report was generated by the program amavisd-new at host mail.emailnw.co.uk. Our internal reference code for your message is 16446-01/V3_vqObHzpeD

INVALID HEADER: DUPLICATE HEADER FIELD

Duplicate header field: "Content-Type"

Return-Path: Keyop@bbs.magnum.uk.net From: "Keyop" Keyop@bbs.magnum.uk.net Message-ID: 6043ED2B.356@bbs.magnum.uk.net Subject: screenshot.bmp

DUPLICATE HEADER FIELD

The RFC 5322 document specifies rules for forming internet messages. Section '3.6. Field Definitions' specifies that certain header fields must not occur more than once in a message header section.

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Please let me know if you need any more info.