Zimbra has released a patch for Zimbra Collaboration 8.6.0. You can find the full release notes and upgrade instructions on Zimbra.com.
With this patch, comes an official assessment of Zimbra web client’s Mail App accessibility (as tested by the W3C), which will be an area of continued investment and have significant improvements in Zimbra Collaboration 8.7. In addition, there are a number of bug fixes and product enhancements.
In the coming months we will provide more information on Zimbra Collaboration 8.7.
To download the patch please visit: https://www.zimbra.com/downloads/zimbra-collaboration; patches can be securely downloaded and verified via MD5 or SHA256 checksums.
*Update: minor clarification change related to MD5 and SHA256 checksums