In our O365 Tenant Personal Mail Files maximum capacity (including Archives) is 99 Gigs 

Shared Mail Files maximum capacity is 50 Gigs. 
 
 
Shared Mail files can have 1 (only 1) license added at full cost to expand capacity to 99 gigs. 

 
Once the maximum capacity has been reached the mail file will no longer send or receive new information. 

The THQ IT recommendation is to (before maximum capacity is reached) export the mail file to a .pst  (.olm for MAC) 
 
Exporting to a .pst (.olm) saves the entire mail file to a single Outlook File. 

 
On a PC: 
 
1. Open Outlook 

2. File/ Open & Export 

3. Import/Export 

4. Export to a File 

5. Outlook Data File (.pst) 

NOTE:  Exporting to a .pst does not delete/remove existing entries from the active mail file. 
Once you have confirmed that the new .pst works (by importing it or using it on a different computer) 
You must manually delete entries from the active mail file.    
 
Now is the time to “retain” items you wish to keep.  For example, if you want to keep the last year’s worth of data, use the sort features to arrange by date and remove only what you wish to remove.  The exported .pst will have ALL of the mail file/calendar etc. 
 
 
 
On a MAC 
 
1. Open Outlook 

2. File / Export 

3. Select the items you wish to export 

4. Continue 

5. Name the file and save.