It's easy, if your mailx command supports the -a (append header) option:
$ mailx -a 'Content-Type: text/html' -s "my subject" [email protected] < email.html
If it doesn't, try using sendmail:
# create a header file
$ cat mailheader
To: [email protected]
Subject: my subject
Content-Type: text/html
# send
$ cat mailheader email.html | sendmail -t
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