I have to dissapont you - I use 4.3.4, but on the other hand the error is not the same like the one above .

I do get "Object reference not set to an instance of an object", as an  Page Load Exception (on the page where calendar module is located), sometimes.

But the SMTP error is the following:

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider:
ExceptionGUID:
InnerException: The SMTP host was not specified.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: System.InvalidOperationException: The SMTP host was not specified. at System.Net.Mail.SmtpClient.CheckHostAndPort() 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: NTXPW19

and.. it is the one that happens when the Event scheduler is triggered.

So the problem could be a different one...and it's a bit wird as SMTP is specified and in general works and sends emails.
Some other modules do not have a problems with sending emails.

Could it be an x64 bit issue? Althougth everything runs in 32 bit mode.

thanks a lot for a quick answer
Goran