This might be just me, but after upgrading to 1.5.20 the tags specified in the body of the email (i.e. tags: foo, bar) are no longer saving to the post.
Reverting back to 1.5.19 resolved the issue.
After reverting to 1.5.19, I accidentally overwrote postie-functions.php with the newer version, which reproduced the error. Looking at the differences between the versions of this file, the wp_set_post_terms in the PostToDB function on line 905 jumped out as a possible culprit.
I had some issues with the custom taxonomies that weren't reproducible, so it might just be me, but thought I'd give a heads up.