We’ll let the experts explain this one.
Visit Microsofts's Create a contact group or distribution list and Manage a contact group pages for more information.
Office 365 imposes a Message Recipient limit of 500 recipients for distribution lists. This is the maximum number of recipients allowed in the To, Cc, and Bcc fields for a single email.
If you need to send a message to more than 500 recipients, a Sympa distribution list should be used to send the email. When an email is sent to a Sympa list, Office 365 interprets the recipient limit as one recipient even though the list could include thousands of email addresses.
The Outlook web client only allows one address to be added to a Contact List at a time; you can't paste a list of addresses into a Contact List. You can paste a list of addresses into an Outlook desktop Contact Group.
If you are copying and pasting an email list into a contact group from Word or Excel, please use column format.