1. SMTP Settings/Mail Server misconfiguration/Mail Server Security. Tried in localhost install of DNN 5.4.4 sending newsletter to recipients in 4 domains with SMTP settings configured for basic authentication. Sender was admin user in same domain but not same username/email account specified in SMTP settings which was that of host user. All recipients received newsletter. Admin user received email summary, Host user received SMTP Test Email. Repeated test after changing sender email to one outside of domain specified for SMTP server. Only email received was SMTP Test Email by the host user. No errors were logged.
2. Host Settings Bug in DNN 5.5.0.336. Repeat of same test in DNN 5.5.0.336 Beta using same SMTP configuration in Host Settings. This resulted in NO emails being sent including the SMTP Test Email. Errors displayed and logged regarding sender not allowed to relay and  recipient being graylisted. Traced this down to being related to Host Settings update error I had reported in Gemini as DNN-13046. In Services\Mail\Mail.vb at line 403, if basic authentication has been specified in Host Settings, the following code is to be executed:
   
   

   Case "1" ' basic
        If Host.SMTPUsername <> "" And Host.SMTPPassword <> "" Then
                 smtpClient.UseDefaultCredentials = False
                 smtpClient.Credentials = New System.Net.NetworkCredential(Host.SMTPUsername, Host.SMTPPassword)
        End If
   
   However, due to an improper fix in DNN 5.5.0 to prevent host settings passwords from being exposed to token replacement, any setting containing the string "password" has been stripped from the host settings dictionary. Host.SMTPPassword will always return empty string with the result that SMTPClient credentials are not properly set. My mail server sees that as an attempt to send without authentication.
   
   This issue is marked as Closed-Duplicate (of DNN-12548) which while having a related issue description is showing the checked in fix as one that is totally unrelated to the issue. Have got to ask Charles about that as I don't believe root issue has been fixed.

3. High CPU utilization/hang/timeouts all appear releated to the regex used to set the body HtmlFormat to either Text or Html. I had tried that same regex when working on the Feedback module and found similar performance issues when the email body was long or complex or it failed to detect Html Markup completely.

Regarding the sending of the SMTP Test Email, this is sent out immediately through a direct call to SendMail by the newsletter module when Asynchronous mode (the default) is selected. Other emails including the summary report are then processed on a separate thread. When sending newsletters with Synchronous mode selected, the SMTP Test Email is not sent. The sending of the Test Email has nothing to do with whether or not there is a SMTP error.