Are you including the SASL class?
phpclasses.org/sasl
It is very likely that the server is requiring Windows NTLM authentication and it may disconnect if the client does not support it.
The SASL class is responsible for supporting authentication mechanisms. It supports Windows NTLM authentication mechanism. You just need to include the SASL class in your script like it is described in the example.
Alternatively, if your Exchange server is in the same server that runs PHP, you may want to try the MIME message class that comes with a sub-class specialized in dropping messages in the Pickup folder of Exchange. It does not require any SMTP connection and so it does not need to authenticate and is even faster.
phpclasses.org/mimemessage