How can I send bulk email?
In general, Office 365 should not be used for bulk email delivery. Attempting to use a general-purpose email system for bulk mail delivery impairs system performance and delays delivery of other message traffic. By default, messages sent via Office 365 can have a maximum of 1000 recipients. ITS provides a dedicated Group Email service which is designed to deliver bulk messages with minimal impact on normal messaging systems. If this service is not suitable for your bulk delivery needs, please consider breaking your recipient list into smaller groups.