This might help, but is not a good solution, from my point of view.

Imagine the following not so seldom combination:

I have my Web running at home, published to the rest of the world with a dynamic DNS-Provider, eg: www.myweb.dynaccess.de

My Maildomain is at a real hoster (1&1) in my case and the addresses are looking like: user@myweb.de

So when i change the admin to admin@myweb.dynaccess.de there won't be any mail at all, as at my home-server is no e-mailserver or even relay installed.

When in remember the companies i worked so far they will have the same problem. They will have an official web-address with all the mail-addresses in that domain and the Intranet running DNN runs under a subdomain or even totally unconnected to the outside domain.

How to fix it then?

When a user tries to get a password reminder from my 4.9.0 site, they get the following:
Password could not be sent at this moment. Please try again later or contact the site administrator.
 

The administrator account has a valid email address.  The same email address is used in the host setting's SMTP settings.  The "test" in the SMTP settings returns a "email sent seccessfully" message, so it should work for the site.

The error in the log viewer shows:

AssemblyVersion: 04.09.00
PortalID: 0
PortalName: xxxx
UserID: -1
UserName:
ActiveTabID: 36
ActiveTabName: Home
RawURL: /Home/tabid/36/ctl/SendPassword/Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.xxxx.org/Home/tabid/36/ctl/SendPassword/Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 7b4b9377-63bd-4e14-bc39-c7219bb756e3
InnerException: Mailbox unavailable. The server response was: Authentication is required for relay
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Net.Mail.SmtpTransport.SendMail
StackTrace:
Message: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: Authentication is required for relay 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, String ReplyTo, MailPriority Priority, String Subject, Mailformat Bodyformat, Encoding BodyEncoding, String Body, String[] Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)
Source:
Server Name: DEDI457