I see questions and issues coming up about delegate access with EAS/ActiveSync quite often. If you are dealing with an issue with ActiveSync doing delegate access, then you need to understand that EAS was not designed nor tested for doing delegate access. Because of this it is not supported. There are some who have devices doing EAS delegate access which have some success in it working, however such functionality can break after any update or upgrade involving Exchange.
I work with the EAS client licensed developer customers who write the mail clients for devices which use EAS, so I want to provide some of the same information to you as I do them. From Exchange 2003 SP1 when EAS 2.5 became the first EAS version to have a licensing option for client side development through and including Exchange 2016/EAS 16.0, there has never been any supported functionality for delegate access from the developer or product side of support. Exchange/Office 365 also does not support delegate access with EAS. Exchange was never written for nor tested for delegate access with EAS. I don’t know if or when this limitation may change.
If you have issues with delegate access, then you should be discussing the issue with your device maker or if you’re an EAS client developer licensee then you should open a support case and pursue a Design Change Request (DCR) to request the specific functionality you are wanting in a future release of Exchange.
Please read the following:
Exchange ActiveSync does not support shared mailboxes or delegate access.
Please read the following blog article:
What Does “Unsupported” Mean?