I've built the last few months a few websites in Orchard and in all of them I’ve successfully implemented contact forms and other functionality which uses the mail functionality of Orchard.
Unfortunately with the website Quebble (also Orchard) we had problems to get this working and we always received a "Authentication Failed" from the SMTP server.
I was really confused why this wasn't working because I’ve made other sites with email functionality in the past without any problems.
In the beginning i thought that the credentials were maybe wrong but I’ve testing them in Outlook and they we're just fine! Today I’ve found out why this was going wrong, because I’ve found a little bug in the email functionality of Orchard which implemented the UseDefaultCredentials property not totally right. See my CodePlex created issue ticket for details:
and the official
BTW: This shows once again how powerful and nice it is to user OpenSource software! Because of the great developers of Orchard I’m sure this little glitch gonna be fixed in no time.