Use other font in restricted area won't compile

The problem is in the fact that \email{x} eventually does

\href{mailto:x}{x}

and \myfont in the first argument to \href is wrong. You can work around this with some trick, but the link will be invalid.

\documentclass[11pt,a4paper,sans]{moderncv}
\moderncvstyle{classic}
\moderncvcolor{blue}
\moderncvicons{awesome}

\newfontfamily\myfont[]{Arial}
\protected\def\myat{{\myfont @}}

\firstname{John}
\familyname{Doe}
\email{someName\myat me.com}

\begin{document}
\maketitle
\end{document}

Here is a better work around that produces a valid hyperlink. Now \email has an optional argument for specifying formatting instructions, so \maketitle will do

\href{mailto:[email protected]}{someName{\myfont @}me.com}

which is legal.

\documentclass[11pt,a4paper,sans]{moderncv} 
\moderncvstyle{classic}
\moderncvcolor{blue}
\moderncvicons{awesome}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\makecvtitle}
  {\emaillink{\@email}}
  {\expandafter\emaillink\@email}
  {}{}
\renewcommand{\email}{\@dblarg\fry@fix@email}
\def\fry@fix@email[#1]#2{\def\@email{[#1]{#2}}}
\makeatother

\newfontfamily\myfont[]{Arial}

\firstname{John}
\familyname{Doe}
\email[someName{\myfont @}me.com]{[email protected]}

\begin{document}
\maketitle
\end{document}