[nSLUG] [Somewhat OT] MTA changes header, mssg body lines. What?

Aaron Spanik a.spanik at ns.sympatico.ca
Tue Feb 15 23:51:05 AST 2011

On Tue, 15 Feb 2011 00:47:37 -0400
mspencer at tallships.ca (Mike Spencer) wrote:

> Question for mail gurus:
> Experimenting (successfully) with sending MIME multipart/mixed email
> from a script, I see that one of the MTAs involved (appears to be
> Eastlink) changes the boundary string definition I create in lines like:
>    Content-Type: multipart/mixed; boundary="$boundary" ;
> where $boundary is some string I made up that is unlikely in text or
> base64 data [1] and is compliant with the BNF spec in RFC 1521.
> The instances of the boundary string where it constitutes a boundary
> within the body of the message are also changed correspondingly.

This suggests it's being done very intentionally (i.e., so as not to
prevent the mail from being decoded and read at the other end.
> Is this usual?  Unusual?  Regarded as acceptable?  Deprecated but
> tolerated?  Weird?

Most likely regarded as unnecessary, possibly gross, and maybe even
wasteful, but generally acceptable.  Whatever Eastlink's mail system is
using to scan the mail for viruses and/or SPAM is the likeliest
culprit. The reason you're MIME encoding in the first place is because
what you're trying to send to the other end isn't (all) naturally 7-bit
ASCII text (the encoding technically still at the heart of the SMTP
standard).  In order to scan it, the scanner has to decode it to
whatever its declared natural form is and scan that.  It can then
either rebuild the message, which it appears to be doing, or it can
pass on the original.

It's also possible that you're sending 8-bit MIME using the 8BITMIME
SMTP extension and the server is playing it super safe by reducing it
to 7-bit before passing it on to whatever the next hop happens to be.

Aaron Spanik
a.spanik at ns.sympatico.ca

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