Dim rk As Microsoft.Win32.RegistryKey
rk = Microsoft.Win32.Registry.LocalMachine.OpenSubKey("SOFTWARE\Clients\Mail", False)
Dim DefaultMailClient As String
DefaultMailClient = rk.GetValue("")
Dim ShellStr As String
rk = Microsoft.Win32.Registry.LocalMachine.OpenSubKey("SOFTWARE\Clients\Mail\" + DefaultMailClient + "\Protocols\mailto\shell\open\command")
ShellStr = rk.GetValue("")
Dim args As String
Dim mailbox As String = "latyn@mail.ru"
ShellStr = ShellStr.Substring(0, ShellStr.IndexOf("%1")).Replace("""", "")
If ShellStr.IndexOf("/") >= 0 Then
args = ShellStr.Substring(ShellStr.IndexOf("/"))
ShellStr = ShellStr.Substring(0, ShellStr.IndexOf("/"))
End If
args += "mailto:" + mailbox
Process.Start(ShellStr, args)

Другой способ попроще:

Dim ShellStr As String
Dim mailbox As String = "latin@mail.ru"
ShellStr += "mailto:" + mailbox
Process.Start(ShellStr)