Woah, woah woah. People are suggesting obfuscators to help hide the credentials listed in the source code, I would not suggest this. Aside from simple sniffing, most of the time obfuscated applications can be deobfuscated easily. The safest method to send emails from an application would be to contact an online php script to send the email (do this by signing up for a free web hosting account somewhere, and copy pasting an email script with GET or even POST variables). If you can't be bothered to do that, I would suggest that you have users insert their own credentials on run time.
What are you attempting to build as a whole of your application? It's possible that you are going the wrong way using SMTP in your app (as there are few instances it should be needed).