Duplicate Email Headers in Newsletters

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I had previously noticed that I will sometimes receive an email rejection for newsletter recipients with a warning that I had exceeded some maximum number of headers in the email message. As the size of my newsletter distribution has grown, the problem has become worse.

 

After some study, I have figured out that the newsletter generator creates a duplicate pair of "Content-Type:" & "Content-Transfer-Encoding:" headers for each recipient in the distribution. Therefore, if I have 400 addresses in my newsletter distribution, each individual message will contain 800 duplicate headers of these types. Of course this causes problems with some email servers, email clients, spam filters, and anti-virus programs.

 

Has anyone else observed this issue and found a way to stop the generation of these duplicate headers? There are several contributions that will break the newsletter distribution into smaller batches, which will fix the problem, but I would like to get the root cause if possible.

 

Thanks