cache exchange mode, online mode, outlook

we had outlook cache mode enabled through group policy in our organization , but due to disc space issue we  disabled cache mode in group policy under user configuration -administrative template- office 2016 - outlook options

we have outlook 2016 and mailboxes in office 365, we dont have any hybrid environment

after disabling cached exchange mode  users are experiencing  with slowness in opening email, switching between in box and sent items, scrolling through inbox

i see under mail icon, cache exchange mode not checked and greyed out but doesnt show on line mode written as well

i deleted my ost file and i am connected to office 365  but my outlook is also experiencing slowness in surfing through emails


anything i should check or i am missing something
pramod1Asked:
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yo_beeDirector of Information TechnologyCommented:
are your mailboxes on Office 365 or on-prem?
timgreen7077Exchange EngineerCommented:
Nope, since you are no longer in cache mode you will experience slower responses from outlook when navigating through the mail. since you are in the cloud you are connecting to an exchange mailbox database which could be hundreds or thousands of miles away from your users and network and everything is going over the internet. There actually isn't anything you will be able to do to resolve the slowness, that is the reason outlook uses cache mode. you will need to enable cache mode again if you want performance improvements. cache mode is local and syncs to exchange server so performance is 100 times better, non-cache mode connections directly to exchange server DBs and because of distance and internet performance is substantially slower. non-cache mode is fine if exchange was on-prem and local, but since in O365 there isn't really anything you can do to resolve the slow responses. This will affect all your users and your internet speeds since it constantly queries exchange for everything.
pramod1Author Commented:
mail box is in 365
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yo_beeDirector of Information TechnologyCommented:
There's your answer as the previous comment stated.

What if you instituted CEM with a time limit of 6 months
That should help.

You can also enable archive mailbox which will reduce the size.
https://docs.microsoft.com/en-us/office365/securitycompliance/enable-archive-mailboxes
pramod1Author Commented:
you said cache mode is local and sync to exchange server , so how  performance was fast in our outlook when we had exchange cache mode enabled when we dont have any on premise exchange server.
timgreen7077Exchange EngineerCommented:
its fast because you are basically connecting using your local ost file and it syncs data back and forth to exchange, but with online mode you are connected directly to exchange and there is no sync but live transactions constantly, so you get a performance hit because of distance. yes you mailbox is in O365 but your outlook client isnt, it may be 1000 miles away from you mailbox DB in O365. Cache mode recommended for better performance.
pramod1Author Commented:
So what is the solution now

Online mode is slow cache mode is occupying space so what should I do
pramod1Author Commented:
What is CEM
Mal OsborneAlpha GeekCommented:
When running in Cache mode, a copy of the entire mailbox is maintained on the local PC, in the .OST file. When not cached, everything remains on the server.

This means that if you open a 10Mb PDF file attachment for instance, in cache mode it is retrieved from the .OST file almost instantly, but in non-cached mode it has to be dragged down from across the internet from the Office 365 server.

Conversely, if you forward an email with a 10Mb .PDF attachment to 20 fellow users, in cache more 10Mb of data goes up to Office365, and 200Mb comes back, as each user syncs their .OST. IN this instance, the email will be slow to get to its destination. IN non cached mode however, the emails are just send inside of Office 365, so recipients will see them very quickly.

In general, cached mode is preferable, as it more frequently performs operations faster. If, however, you have machines that numerous users log onto occasionally, it is often better to run non cached.

In a terminal server environment, it is common for most local disk space to be used by .OST files. General way of dealing with this is more disk space, although switching a few users to non cached mode might be useful to "buy time"
timgreen7077Exchange EngineerCommented:
I agree with @Mel, but slight correction to his comment, the entire mailbox is only cached if you dont limit the dates to cache, but in order to keep the ost size down you normally only cache a few months and not the entire mailbox.
pramod1Author Commented:
Just 1 last question

How OWA is fast as compared to outlook client which is slow on online mode

Any difference
timgreen7077Exchange EngineerCommented:
OWA is located on the web, so the benefit from that is non cache and fast. The reason it is fast is because it is web based so the server you are connecting to for OWA is most likely located in the same data center with your mailbox. it is web based so you are not pulling down mail to and from your local computer. it is all happening on the web based app OWA and not local, where as outlook is local.

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pramod1Author Commented:
I see my Ost file not being used any more but my share point ost is being used anybreason
pramod1Author Commented:
Share point .pst is being used
timgreen7077Exchange EngineerCommented:
I don't know about sharepoint, but you outlook ost file will not be used if you are no longer in cache mode.
yo_beeDirector of Information TechnologyCommented:
CEM = Cache Exchange Mode
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