Make sure, that the ip address you are calling from is not black listed. This is e.g. the case if the ip is a dynamic IP. Also make sure, that you have a correct reverse dns lookup for your ip address, matching the hostname of your from adress. Alternatively use a SPF DNS record entry matching the host part of the from address.In case of a blacklisting, the target mailserver may respond with an SMTP 554 or just let you run into a timout.