The "correct" way to use a connection at runtime is to retrieve it from the connections collection, and call the AcquireConnection method. This gives you a valid connection object.
In the case of the SMTP Connection that object is actually a string, the connection string, e.g. SmtpServer=mail.domain.net;UseWindowsAuthentication=True;EnableSsl=False;
Code Snippet
' Get the SMTP connection, the result is string rather than a actual SMTP connection type
Dim smtpConnectionAsString As String = CType(Dts.Connections("SMTP Connection Manager").AcquireConnection(Nothing), String)
You can take the more direct approach, and to be honest it will work, and just retrieve the SMTP Server property directly, e.g. mail.domain.net
Code Snippet
' Get the SMTP connection's server property value
Dim smtpConnectionManager As ConnectionManager = Dts.Connections("SMTP Connection Manager")
Dim smtpServer As String = CType(smtpConnectionManager.Properties("SmtpServer").GetValue(smtpConnectionManager), String)