Note that your filter_data function is being called twice: once for write_data and once for mail_data. If this is an expensive function you should consider running it once, storing the results, and pass those results to each of the two functions.

If your main functions are too long to post, consider just reading through them and check your assumptions and error checking. What happens if it can't get a response? What happens if the response it gets doesn't follow your expectations? Are you looping and using a variable to determine when the loop should end? If so, are you sure this variable is allowing the loop to exit when unexpected things happen?

In the past, I've occasionally used 'strace' (or 'truss' or 'ptrace', depending on your OS) against a process stuck like that. That usually lets me see what system calls it's trying to do (if any). Sometimes this points me to the right place in my code.