Postmark, an email sending service, experienced delays in sending emails last week due to issues with their primary data store, MongoDB. The company had previously tried using CouchDB but found it to be less performant than MongoDB and eventually reverted back. To resolve the issue, Postmark is working with 10gen, the developers of MongoDB, and upgrading its infrastructure at Rackspace to improve performance. They are also migrating their replica sets to better hardware to reduce slower I/O from EC2. The delays were caused by high load on MongoDB leading to iowait on the servers, which slowed down queries and ultimately delayed sending emails. Postmark is taking steps to optimize and upgrade MongoDB to prevent such issues in the future.