$ cat sample.txt | perl -wnl -e '/\.X\-Intermail\-Unknown\-MIME\-Type\=unparsedmessag(e.*)/s and print $1;'

##</code><code>##

$ perl -0777nle '/\.X-Intermail-Unknown-MIME-Type=unparsedmessage(.*)/s and print $1;' sample.txt