K2 E-mail notifications take long to be sent out, the MSMQ public queue starts to fill up with email messages that aren't being sent to users in a timely fashion.
This results in a scenario where the MSMQ will fill up during the course of the day, which may cause some users to only receive their notification emails the next day.
Our developers identified a problem within the product, and provided a codefix for our 4.6.11 clients. This issue has been resolved in the latest version of K2.
If you require a 4.6.11 codefix, please contact K2 Support and they will be able to assist further.