What to do if you can't send mail from Gmail to docomo
The reason why the email cannot be sent is on the docomo side.
rejected by the server for the recipient domain Docomo.ne.jp
If the message above is included, the person you want to send email to is rejecting it.
Internal parse error: Illegal envelope To: address
If the message above is included, there is a problem with the email address of the person you want to send email.
See the following articles for details.