Postie creates a placeholder post with the title "tmptitle" when receiving your emails so that it has a place to store any attachments before starting to process the content of the email. So if something goes wrong these get left behind.
Things that might go wrong in the order of likelihood:
PHP ran out of memory. There should be a message in the web server logs. This is often caused by a very large attachment in an email. You may need to log into the email account and delete the message. See this article on how to adjust the amount of memory allocated to PHP.
PHP ran out of time.There should be a message in the web server logs. This is typically because there are either a lot of emails (100s) or your email server is slower than average. You can use the "Maximum number of emails to process" setting to limit how many emails are fetched at one time. You might also be able to increase the amount of time PHP allows a script to run by changing the max_execution_time PHP variable. Contact your host for instructions.
A bug in the image library. This one is frustrating because there is no error message anywhere, but some hosts seem to have a buggy version of the software that resizes the images.
A bug in Postie. Doesn't happen very often, but it is possible. If this is the cause you will see a PHP error message in your web server logs. Please post the error message in the support forum and I will get it fixed.
A bug in a Postie AddOn. If it is my AddOn send a message to help@postieplugin.com. If it is your AddOn it is up to you to fix it :-) If you're having trouble post in the forum and I will try and help.
A bug in another plugin. Most plugins don't think about posts being created somewhere other than the post editor so sometimes there is an incompatibility. In general most plugin authors want their plugins to work for you and will create an update to fix things. I am always happy to work with another author to ensure things work well together.