We're having problems with email notifications to both the users and admins when activites take place like registering, deleting a user, removing user from a group, etc.

We have verified the DNN settings are working.  When we do the 'test' link on the SMTP setting from HOST  SETTINGS we get the test email. 

Here is a sample of the log entry from a failed message triggered by removing a user from a group:

AssemblyVersion: 04.03.04
PortalID: 0
PortalName: SignZUp
UserID: 1
UserName: host
ActiveTabID: 42
ActiveTabName: User Accounts
RawURL: /Default.aspx?tabid=42&filter=All&currentpage=1&PageRecords=10
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.mysignzup.com/Default.aspx?tabid=42&filter=All&currentpage=1&PageRecords=10
UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: d052d26a-8d8c-4403-bfa7-4698d47fbd7f
InnerException: Mailbox unavailable. The server response was: <mikel@clayhillproperties.com> No such user here
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Net.Mail.SmtpTransport.SendMail
StackTrace:
Message: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: <mikel@clayhillproperties.com> No such user here at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, String Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword)
Source:
Server Name: DEDI469

 

I am seeing this in my 8.0.3 install. In my case the error message is bogus and I know the mail box does exist because I can successfully send to it from a feedback module. Any mail sent from the registration process or by other modules causes the exception:

2016-09-10 07:53:56,370 [WIN5124][Thread:34][ERROR] DotNetNuke.Services.Exceptions.Exceptions - System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: No such user here
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at DotNetNuke.Services.Mail.Mail.SendMailInternal(MailMessage mailMessage, String subject, String body, MailPriority priority, MailFormat bodyFormat, Encoding bodyEncoding, IEnumerable`1 attachments, String smtpServer, String smtpAuthentication, String smtpUsername, String smtpPassword, Boolean smtpEnableSSL) in D:\Users\bitzb\Documents\visual studio 2015\Projects\School Council\Dnn.Platform-8.0.3\DNN Platform\Library\Services\Mail\Mail.cs:line 139