Change the dialplan to:
[send-email]
exten => _.,1,NoOp(Entering user defined context [send-email] in extensions_custom.conf)
exten => _.,n,set(emailaddresss=${STRREPLACE(EXTEN,:,@)})
exten => _.,n,System(echo 'Call from ${CALLERID(name)} at ${CALLERID(number)}' | mail -s 'SUBJECT' '${emailaddresss}')
exten => _.,n,hangup()
Then the custom dial string becomes:
local/email:domain.com@send-email
you replace the @
symbol in the email address with a :