Steve Clay's plugins:
Plaintext only: This was not designed to work with HTML e-mail which other plugins may provide. YMMV.
Note: This plugin can alter most notification emails, but (as of 1.8.8) not e-mails sent directly with the elgg_send_email() function. For that you would need to apply this patch to the Elgg core (I hope this will be native in 1.8.9 but cannot guarantee that!).
i'm wondering if this will merge with html emal handler plugin:
or is this plugin just for plain text format?
It was designed to extend the plaintext e-mails sent by Elgg. You could probably change the subject/sender name using this, but header/footer content would probably end up outside the BODY of an HTML e-mail message (possibly breaking it).
If this were your only plugin, HTML in the header/footer would just show as raw markup.