I was having the same problem.  Running DNN 5.4.4.  Changed all Admin Host and SMTP email addresses to the same one.  No effect.  Pulled most of my hair out - worked through the night!  Found a solution this morning.

SOLUTION FOR US
We are using Australian Language pack.  The entry in the "Resource Name: EMAIL_USER_REGISTRATION_PUBLIC_SUBJECT.Text" contained some extra characters - paragraph returns in this case.  Removing these extra characters solved the problem.  The error message actually was pointing out that this was the problem all the time: "The specified string is not in the form required for a subject."  The setting is found at ADMIN/LANGUAGES and then choose your active language and edit at the Portal/Host/System level.  We edited at the portal level and will update the others later.

Hope this helps other with this vexing issue - Peter Smith

In DNN 05.06.02 my email server was refusing to send the sent confirmation email because the From field of the email is the email address of the portal admin which was not acceptable. Changing my portal admins email address to the same as the super user email address fixed the issue for me.

Most email servers don't like to spoof the From field completely, for instance, my godaddy email server will allow the From field to be anything@Mydomain.com but will not allow anything@google.com

Host Email: thisone@packershome.com
Super User Email: thisone@packershome.com
Portal User Email: thisone@packershome.com


I'm not sure if this is a feature or bug, but I'm not exactly a fan of being required two power users needing to have the same email address. However, I'm please to say, it resolved my registration issues.


Thanks for the posts.

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