Microsoft Outlook emails are stored in a single database file with an extension of .ost or .pst. Various Outlook versions have different .ost/.pst storage limits. When the .pst or .ost file reaches it's storage limit you will get an error "Your mailbox has reached its maximum size limit at which you cannot send or receive any more content. Empty your deleted items folder from Outlook or delete messages from your mailbox to create more space." This can be fixed...read on.
What is an .OST or .PST file?
Outlook Data Files (.ost) are used when you have an Exchange account and want to work offline or use or use the default Cached Exchange Mode. This type of data file is also used for accounts that you set up with the Outlook Connector for Outlook.com (formerly Hotmail). Outlook Data Files (.ost) are always copies of items that are saved on a mail server and don’t need to be backed up like Outlook Data Files (.pst).
Outlook Data File (.pst)
An Outlook Data File (.pst) contains your messages and other Outlook items and is saved on your computer. When you use POP accounts, all of your email messages from your mail server and save them on your computer.
Note: In Outlook 2013 and earlier versions, IMAP accounts also used an Outlook Data File (.pst). In Outlook 2016 and Outlook for Microsoft 365, IMAP accounts use Offline Outlook Data Files (.ost).
Offline Outlook Data File (.ost)
Most other account types, such as IMAP accounts, Microsoft 365 accounts, Exchange accounts, and Outlook.com accounts use an Offline Outlook Data File (.ost) file to store a synchronized copy of your mailbox information on your local computer.
.OST & .PST Limits
The various Microsoft Outlook versions come with specific data storage limits. Outlook 2010, 2013, and 2016 have maximum limits of 50GB. Whereas, Outlook 2003 and 2007 have a maximum data storage limit of 20GB.
How To Fix Outlook Error "Your mailbox has reached its maximum size limit at which you cannot send or receive any more content. Empty your deleted items folder from Outlook or delete messages from your mailbox to create more space."
You have two options:
1)delete or archive email messages or
2)increase the size limit of the .ost/.pst file.
Option 1 is self explanatory, but what if you prefer option 2?
Adjust the .OST or .PST file size limit in Windows Registry
For this option we will have to create Windows registry entries.
Note: If you have are not familiar with editing the registry then this option is NOT for you. If you feel brave and what to go ahead then make a BACKUP of your registry first.
Before you start, make sure Outlook is closed.
Note: Check your version of Outlook and match it to the option below.
Click Start, click Run, type Regedit, and then click OK.
In the left pane, expand the following registry key based on your version of Outlook:
For Outlook 2016
For Outlook 2013
For Outlook 2010
For Outlook 2007
For Outlook 2003
Right click Outlook, point to New, and then click Key.
Type PST, and then press ENTER. (The screen shot for this step is listed below).
Right click PST, point to New, and then click DWORD value. (The screen shot for this step is listed below).
Type MaxFileSize, and then press ENTER twice.
In the Edit DWORD Value window, choose Decimal and type the value in the Value data box, and then click OK.
Note 1GB=1*1024*1024*1024byte; 1MB=1*1024*1024byte; 1KB=1*1024byte, the example below is for a 100GB size limit with a 95GB warning limit.
Repeat above steps to create another DWORD MaxLargeFileSize.
Repeat steps above to create another DWORD WarnLargeFileSize, and then close the registry.
Close Registry Editor
You can now open Microsoft Outlook.