The default mailbox size limit for USI email (Employees, Students and Alumni) 50 GB. This means that when your mailbox contains 50 GB of "stuff" you will be prevented from SENDING email.
FYI: 'mailbox' means all types of items: Mail, Calendar, Sent, Tasks, Deleted items.
The problem is not necessarily the NUMBER of items, but the size of the items your mailbox contains. Mail items with photos, avi, mov, pdf, ppt are typically very large in size.
What can you do if your mailbox is full? The easy answer is Delete items.
If you need help cleaning up your mailbox click here.
FYI: The largest SINGLE message you can send/receive with USI Outlook is 25MB.
Outlook Web App (OWA) in Office 365 offers an easy way to “attach” a document as a link to where it’s saved on OneDrive for Business, a great option for sharing large files with your colleagues.