mail aliases treated as regexp causes mismatch

Currently a regexp is created for every alias in configuration file. However
special characters are not escaped which causes a mismatch. Mail aliases are
string literals that may contain lots of special regexp chars and those
should be escaped before compiling a regexp.

For instance, one may have an alias like:
a.lot.of.dots@server.com

Gmail aliases are constructed with + sign:
username+anything@gmail.com

which would match 'usernameeeeeeanything@gmail.com' but not 'username+anything@gmail.com' =(

This commit fixes this issue.