Sending email using Smtp.mail.microsoftonline.com

Question

The context:

We’re a small company that does not have an Exchange Server (or anyone dedicated to it) yet we still need to have/send emails.

Answer 1

5.7.1 is not an authentication issue, but a relay issue. In order to prevent anyone from using your server (or account, as the case may be) the smtp server is configured to only allow mail to users outside your domain if it is comming from an authoritive address. Verify that the address you have listed here

mail.From = new MailAddress("xxx@domain.com");

is the same as the one you are authenticating as. Also, make sure that the domain of the address listed is in the authoritive domains list.