The "content" is delivered as "body" by curl, but the envelope continues
after the body and the rest of it should be delivered as header.
The IMAP server can now get 'POSTFETCH' set to include more data to
include after the body and test 897 is done to verify that such "extra"
header data is in fact delivered by curl as header.
Ref: #7284 but fails to reproduce the issue
Closes#7748
