Mesmerize won't send text messages to certain contacts

My Samsung mesmerize started not sending text messages to certain contacts and the only way I realize it is when I ask them if they get my text and they say they never got them. Any ideas on what I could do to fix this problem? Is there any texting apps I could download that would fix my problem?

The firmware on my phone is 2.3.5,, not sure if that helps at all

Thanks in advance