As customers start to transition out of BPOS and into Office 365, each and every administrator should be looking to the Office 365 Service Description documents, which outline the features, functions, and capabilities of these new services. Specifically as users start using EXO365 they may find that the amount of messages, message sizes, etc are not what they are used to seeing and using. This is because BPOS EXO and EXO365 do use different Messaging settings, with a specific callout here.
Example of Change Between BPOS & EXO365
Message Limits in BPOS are set to 1200 messages every 90 minutes, while EXO365 limits are set to 1500 per 24 hours.
For additional Messaging Settings within Exchange Online 365, you should review the Service Description to fully understand these differences and/or changes: