How big and long can you go with Outlook?
As usual with Microsoft Office, there are no simple answers to some of these questions.
Outlook itself has few size limits because the PST/OST system has been adapted over time to expand way beyond current needs.
But Outlook can appear to be limited but it’s really obeying external factors like the limits set by mail hosts.
Modern Outlook is very accommodating. Most people will never have to worry about growing beyond Outlook storage limits.
There are limits on incoming and outgoing messages but, with one exception noted below, those are restrictions by the mail hosting service not Outlook.
Let’s start with a common question from the Outlook-curious.
The Past and Future Calendar
Outlook’s Calendar can handle dates over almost 2,900 years.
Appointments and Tasks can be dated from 1 April 1601 through to 31 August 4500
That should be enough for most people <g>.
The year 1601 start may be partly because of the switch to the Gregorian calendar that started adoption from 1582. Dates before 1582 have to negotiate the difference between Julian and Gregorian dates which would be too much for Outlook to handle.
Largest Outgoing Email
Maximum outgoing mail size isn’t just set by Outlook. The size of messages is mostly governed by the email hosts – sending and receiving. But Outlook has its own hard limit for outgoing messages that are sent via SMTP or IMAP.
Each email host has a size limit on incoming and outgoing messages. For example, Outlook.com and Gmail it’s 25MB.
However, Outlook 2010 and later will stop any outgoing message (SMTP or IMAP) that’s over 20MB. That applies even if the email host allows larger messages. You’ll get the message ‘Attachment size exceeds allowable limit’.
Exchange Server and Office 365 mail hosting are controlled by the administrator. Outlook will use whatever limits are setup in Exchange Server.
Office 365 hosting defaults to 25MB but the administrator can increase that to 150MB.
Other Exchange Server systems will depend on the network admin, so ask them. Broadly, the default for Exchange Server 2016 is 35MB. Exchange Server limits can be complicated by organizational limit, send connector limit and any limit set by anti-virus/anti-spam software.
Largest Item
Each item in Outlook (calendar, contact etc.) can be very big. The attachments to an item can be many and large.
We could not find any indication of limits for an individual Outlook item.
We’ve had Outlook running with a 1GB attachment to a single item and it worked normally. (In fact, Peter forgot the mega-attachment was there for over a year!) That’s not something we’d recommend but it’s reassuring to know that Outlook can handle a lot more than most people will throw at it.
Similarly, there’s no apparent limit to the number of attachments to a single Outlook item. It would be hard to manage a long list of attachments to an Outlook item so it’s not a good idea to have too many. Again, Outlook can handle much more than most people will try.
Items per folder
Outlook itself has no limit on the number of items in a single folder (any type of folder; mail, calendar, contacts). However, there are both practical and fixed limits enforced by mail hosts like Exchange Server or IMAP.
Depending on the version of Exchange Server, there are items per folder limits from 5,000 (Exchange Server 2003), 20,000 items (Exchange Server 2007), 100,000 items (Exchange Server 2010) and a whopping 1 million items for Exchange Server 2013 or 2016.
But those are the designed limits. In practice, having that many items in a single folder would result in a slow Outlook/Exchange Server.
For users linked Exchange Server 2010, you can comfortably have 50,000 items or more in a single folder without Outlook (or Exchange Server) slowing down.
With Outlook syncing to a mail host via IMAP, the limits will be a lot lower than Exchange Server. The exact values depend on the mail host and IMAP software used so check with the hosts for details.
Outlook with POP (where messages are moved from online storage to Outlook) there’s no limit in the number of items in the Inbox or other folder.
Maximum size of PST/OST
The .PST file is the place where Outlook stores all your email, calendar, contacts etc. .OST is the some thing for Exchange Server connections. PST/OST is a large and very complex database file.
Long gone are days when Outlook would ‘choke’ when the PST/OST got near a mere 2GB. Also gone are the early days of Outlook where users would have to trim their PST/OST file to keep Outlook running quickly.
Outlook 2003 quietly changed the PST/OST format to a different, much roomier format. PST/OST’s can now be a massive 33TB (Terabytes) assuming you had the hard drive storage and the need.
Of course, there’s a practical limit. Your computer needs the resources to handle really large Outlook storage. CPU speed is part of that but it’s mostly hard drive speed (SSD drives are great for Outlook) and free memory that makes the difference to a speedy Outlook with a larger PST/OST.
On a modern desktop/laptop computer with 4GB of RAM you should be able to run Outlook comfortably with at least a 5GB PST/OST file; probably much larger. These days, many people have over 20GB of Outlook storage in regular use; which was unthinkable a decade ago.
