need to replace sender's email address

re,

When I get an incoming mail to my(_at_)my(_dot_)com, I want procmail to replace 
sender's e-mail address with his(_at_)my(_dot_)com and 
then forward it to hers(_at_)my(_dot_)com(_dot_)

if anyone could help with the syntax, I would really appreciate it.

thank you.
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