Outlook is using =
as an escape symbol. If X
and Y
are hex digits =XY
must be substituted with the character with ASCII code XY
. If =
is followed by a newline this newline must be removed and the lines joined.
Outlook does this because only a limited range of byte values can be transferred safely via SMTP and bytes outside of this range must be quoted. Also there are limits on the line length, so Outlook by default splits lines that are longer than 75 bytes long. I believe this is called Quotted-Printable
encoding.
Check the Content-Transfer-Encoding:
header in the .eml file and run the file through a decoder before applying your filter. And encode it again after filtering.