With exchange 2003 at least
From here
https://www.autoitscript.com/forum/topic/66691-_inetsmtpmail-mail-failed-with-error-code-50/
You need to use a -1 for the optional parameter
| $sFirst | [optional] string sent before helo for the smtp server connection (by default {SPACE}). To not send any character this parameter must equal -1, some SMTP server required it. |
From https://www.autoitscript.com/autoit3/files/beta/autoit/docs/libfunctions/_INetSmtpMail.htm
So it ends up being
Local $iResponse = _INetSmtpMail($s_SmtpServer, $s_FromName, $s_FromAddress, $s_ToAddress, $s_Subject, $as_Body,”,-1)
Michael Ellerbeck 