Mail in Office 365

Basic Information

Web Interface (URL)
Username The username includes, e.g.
Password Your primary university password (same as to the study system)
Android – recommended client Microsoft Outlook for Android from Google Play
iOS – recommended client  Microsoft Outlook for iOS from Apple Store.
Windows|macOS – recommended client  Outlook. Users have 5 Microsoft Office licenses available, which include the Outlook mail client
Basic Limits/Restrictions
Mailbox Capacity


Mailbox Type Capacity Comment
Office 365 Personal Mailbox 50 GB Users with A3 and A5 Plans have twice the capacity.
Personal mailbox in InSIS 50 MB i.e. 1000 times smaller than Office 365.
Shared mailboxes in Office 365 50 GB


Sending emails from the mailbox is blocked when approaching the limit; incoming emails are rejected when exceeded.

Size of Emails
Message Type Size Comment
An incoming email to school mail 15 MB Including the total size of all attachments.
Emails between Office 365 35 MB It is better to upload the file on OneDrive and send only a link to the file.
An outgoing email from Office 365 to the outside internet mail 18 MB It is better to upload the file on OneDrive and send only a link to the file
An outgoing email from InSIS 15 MB Such a large email will usually not be sent because the capacity of the mailbox is exceeded
Size of the email sent via 15 MB Applies to users with IMAP access to their mailbox

Office 365 – when sent using the Outlook desktop client, the maximum email size increases to 84 MB.

Number of Messages, Number of Recipients in Message
Type of limit Number Comment
Office 365 – maximum of recipients per day 10,000 For all sent messages from one mailbox
Office 365 – maximum of recipients in one message 500 Email addresses in To, Cc and Bcc are counted – maximum of recipients per day 2,000 For all messages sent in 24 hours – maximum of emails per day 500 In one message


More Information

Other limits of Office 365 (Enterprise E1 and potentially E3 limits apply to VŠE).

More Articles and Information
  • Searching for users in Office 365 – how to quickly find a school recipient’s email address in the mail client
  • Old emails are lost – last year or older messages are not visible in Outlook
  • Outlook does not search for all emails – sometimes an email is not found or only a part of emails is displayed.
  • Localization of Folder Names, Time Zone Change
  • Clutter folder and Focused Inbox – two variants of the same feature that helps you automatically sort your Inbox. For users with tens/hundreds of messages per day.
  • Applying rules to inbox messages – for example, you want to automatically move some of your inbox emails to a specific folder.
  • Policies – Automatic Deletion of Emails after a Certain Time
  • Office 365 and Junk Mail (Spam) – How to Filter Junk Mail, How You Can Influence SPAM Filtering.