Users who have upgraded to the latest version of OS X El Capitan may be experiencing problems if they use Microsoft Office.
Microsoft have acknowledged the issue and have said they are working to resolve it.
Faisal Jeelani, who is part of the Outlook for Mac development team said:
We are aware of the issue with Outlook 2011 on El Capitan and of Office 2016 on El Capitan.
We are working on a fix for 2011 and for 2016 we are working closely with Apple to resolve this issue as soon as possible. Unfortunately we do not have any timelines for either fix, please stay tuned and thank you for your understanding.
While Microsoft are working on a fix, it may be best to wait a while before upgrading your Mac to El Capitan.
If you have any questions regarding this, please feel free to contact us.