The default mailbox size for Exchange Online E3 and E5 users is increasing from 50GB to 100GB


Microsoft hasn’t said anything about increasing the default quota for Exchange Online mailboxes from the previous 50 GB limit, so it came as a surprise when the Office 365 Roadmap announced that an increase was on the way. The last change was from 25 GB to 50 GB in 2013. Maybe we will get 200 GB mailboxes in 2019?

15 Replies

Thanks Tony, I do have a recent message in my Admin Center about it with a link to Technet.


Thanks. Looks like the page was updated on December 14.

Anyone know if Outlook OST can handle that much or if you need to be in Online mode?

I knew 2016 could do 50GB... but i still tried to get my users to stay under 20GB like th older cleints required.

I doubt Outlook's OST can handle a fully-synchronized 100 GB mailbox. You will have to reduce the amount of data you synchronize or connect online. Even the fastest SSD will have trouble with an OST larger than 20 GB...

Supposedly the OST and PST files can grow to be very large. By default, Outlook will only support 20 GB PST and OST files. However, if you modify the registry (we use Group Policy to do this, but you can manually do it as well), then you can increase the supported size. We have supported over 100GB PST files (OST files are the same file type).


Here are the registry keys to add (based on Office version), now have fun with that :)



MaxLargeFileSize        REG_DWORD       Decimal Value: 102400 (100 GB)      
WarnLargeFileSize       REG_DWORD       Decimal Value: 102400 (100 GB)      

@Bill Rupp 

thank you for sharing such useful information, I would share ahead this info.

@Tony Redmond 

I have seen Outlook sometimes freezes due to load of emails and it also depends on emails sensitivity and complexity in handling large email files.

I still have Outlook 2013 on my lattitude dell and it is working nicely with 4GB of emails these emails are backed up for 4 years.

i have seen the issue with a person who had 7GB OST file, most of his emails has attached items, i then suggested him to split ost file move to Office 365.


@Colwill I've been using Outlook since Outlook 97 (yes, I am that old). And in that time I've seen the client improve enormously in terms of its ability to handle large quantities of email. Even so, I keep my OST to under 5 GB because I know that even with a fast SSD (I use a Microsoft Surface Book II), it's the only way to ensure reasonable performance. Outlook has its moments and a freeze is still certainly possible (and given the way the Gods of IT work, it will happen soon), but generally speaking, the "keep your OST under 5 GB" formula has worked for me.

I totally agree with Tony. However, I'm not sure if Outlook has been optimized more for using cached mode in regards to large OST files, but it is very apparent that using a large OST file has less performance degradation than using a large PST file. We have supported execs with both large OST files and large PST files. OST performance has been good, where PST has been noticeably slower for some reason (folder enumeration and searches), and the horrors of OST/PST corruptions of the past, I feel, are mostly just in the past (whew). We've had pretty good luck with them over the past 10 years or so.
Some other thoughts... I personally have a total of over 2 million messages in my Outlook view. I keep about one year of mail in my mailbox, syncing all to my OST (cached mode keep all). I have a PST file for every calendar year, and I set outlook to autoarchive everything older than a year to the previous year's PST.
I keep all of my mail, because I refer to it here and there, and I have found that Outlook doesn't really do a good job of quickly searching across a ton of PST files with this much mail, so I use X1 client for my searches. (I'm not a reseller, just a user, but nothing beats it for searches, so I've used it, and we've used it at my company, for maybe 10-12 years).
PS. I've supported Exchange/Outlook and even Exchange client since 4.0 beta. -A special thank you to Tony for publishing all of your awesome books and articles over the years, as they have greatly contributed to everyone's understanding of Exchange!
-Bill Rupp

@Bill Rupp Well, I'm not sure about the books... but in any case, I enjoy writing, which is why I do now.


In any case, when we talk about cached mode, it's important to realize that this is a combination of:

  • Foreground threads (for delivery of new email)
  • Background threads (for "drizzle mode" synchronization of mailbox folders)
  • OST (storage)

Network hiccups can slow the first two; a slow disk will slow the OST. The more data is in the OST, the harder it must work, and the harder it works, the more influence disk speed has on its performance. This is why I observe the 5 GB on SSD rule.


@Tony Redmond  :smile:

Good that you mentioned the foreground vs. background... I guess I forgot to mention that when we have users with large mailboxes/OSTs on high latency networks or heavy travelers (satellite or cell when traveling), we turn off Outlook Hybrid mode for them, because it totally hangs Outlook in cached mode.


Add this key and set value to 1 to disable hybrid and run totally cached mode/sync:




DWORD -  hybrid!localcaching







@Bill Rupp the interesting thing is that says that Fast Exchange Access (which this feature was called) is deprecated in Outlook 2016. However, the registry setting still exists for that version and obviously still works for you. I wonder what's going on?

@Tony Redmond 


Still seems to work with Outlook 2016 and 2019. "Deprecated" vs. discontinued, so maybe it partially works (?) (like my powershell commands that I'm supposedly not supposed to use anymore that still work great, but give me a bunch of annoying yellow-text warning messages). I haven't tried it with the 365 Click to Run yet.  ...There's always some "purple smoke", as my team likes to call it, going on behind the scenes with Outlook!

@Bill Rupp Twenty-year old clients do tend to carry some baggage with them...