While reading through the forum I see many entries for the same issue I faced a couple of days ago. I just answered a user named Amit but then I see there are even more users who are complaining about this topic. Let me address this in a generic fashion below.
The following assumptions apply;
1) Installed WP & 'Post by Email' plugin
2) Configured email provider to accept POP3 or IMAP connections on 995 & 993 respectively, SSL enabled.
3) account contains emails that should be parsed.
If you are doing this locally on your machine / a server you administer & using WAMPSERVER or a similar setup, have a look at the blogpost by Nikunj Bhat. Here he describes the necessary modules that need to be enabled on the PHP & Apache to ensure you are able to access your mail provider programmatically.
If you are experiencing this when your WP instance is based on a shared hosting like for example at Hostmonster or GoDaddy, then you will have to investigate how you can enable the list of modules in such an environment.
One of my sites is at hostmonster, and they are telling me two different things, Support says the ports are open but the FAQ tells me I need a dedicated IP address at their end to enable ports < 1024 AND I can also see that the PHP config is universal for all tenants on that instance, so I doubt I can get the modules enabled only for my instance (unless there is a way to do so and I have not found it yet.)
Hope this helps.