Process only messages with Proofpoint message headers present

[gavanderhoorn]

Headers observed in "proofpoint-ed" message:

X-Proofpoint-Virus-Version
X-Proofpoint-Spam-Details

If those headers are not present, the message has most likely not been butchered processed, so no need to parse and change it.

[gavanderhoorn]

badly outdated/broken, but perhaps still informative:

• https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Thunderbird_extensions/HowTos
• https://developer.mozilla.org/en-US/docs/Thunderbird/Message_View_Examples#section_8
• https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Thunderbird_extensions/HowTos/Common_Thunderbird_Extension_Techniques/Filter_Incoming_Mail

https://stackoverflow.com/questions/162057/how-do-you-insert-email-headers-with-a-thunderbird-extension