documentclass{article}
usepackage{color,hyperref}

catcode`_=11relax
newcommandemail[1]{_email #1q_nil}
def_email#1@#2q_nil{%
  href{mailto:#1@#2}{{emailfont #1emailampersat #2}}
}
newcommandemailfont{sffamily}
newcommandemailampersat{{color{red}small@}}
catcode`_=8relax

begin{document}
email{foobar@gmail.com}
end{document}

documentclass{article}
usepackage{color,hyperref}
    catcode`_=11relax
    newcommandemail[1]{_email #1q_nil}
    def_email#1@#2q_nil{%
      href{mailto:#1@#2}
        {{emailfontdetokenize{#1}emailampersatdetokenize{#2}}}%
    }
    newcommandemailfont{sffamily}
    newcommandemailampersat{{color{red}small@}}
    catcode`_=8relax
usepackage[T1]{fontenc}

begin{document}
email{foo_bar@gmail.com}
end{document}

documentclass{article}
usepackage[colorlinks]{hyperref}
usepackage[T1]{fontenc}
DeclareUrlCommandemail{urlstyle{rm}}

begin{document}
email{foo_bar@gmail.com}
end{document}

documentclass{article}
%usepackage[T1]{fontenc}
usepackage{color,xparse,l3regex,hyperref}

ExplSyntaxOn
NewDocumentCommand{email}{m}
 {
  svend_email:n { #1 }
 }
tl_new:N l_svend_email_tl
seq_new:N l_svend_email_seq
cs_generate_variant:Nn seq_set_split:Nnn { NnV }
cs_new_protected:Npn svend_email:n #1
 {
  tl_set:Nn l_svend_email_tl { #1 }
  regex_replace_all:nnN { cD. } { c{textunderscore} } l_svend_email_tl
  seq_set_split:NnV l_svend_email_seq { @ } l_svend_email_tl
  href{ mailto:#1 }{ emailfont seq_use:Nn l_svend_email_seq { emailat } }
 }
ExplSyntaxOff
newcommandemailfont{sffamily}
newcommandemailat{{color{red}small @}}

begin{document}
email{foobar@gmail.com}
email{foo_bar@gmail.com}
end{document}