Masquerading is cumbersome even in sendmail. Does su to customerservice not work? I've never tried.

The reason for not trying is sendEmail:
If you are not totally bound to mutt consider using a well-tested app that does a lot of mail sending very well, IMO: sendEmail. masquerading is easy.
Software :: SendEmail - Send email with this free command line email client

FWIW - This may sound trite, but running an everyday app under root is normally not an idea to consider.