Fix email

Here are two bugfixes in email sending from OPA, uncovered while having my students work on an OPA project:

- When using the sendmail transport, `-t` needs to be provided on the command line. nodemail.js used to do that, but removed this recently without updating the documentation ( andris9/Nodemailer@b14bf2af9d442fbfd9321595fca321c9db746dac). An issue has been opened to update the doc.
- When using the SMTP transport, authentication is attempted even when it is not requested because the nodemailer.js option was unconditionally added an `auth` field.