Microsoft has put some limits on the accounts to prevent abuse of the service (e.g. sending unsolicited email from their servers), this is not unusual with any provider and for normal business use, these should not be prohibitive in any way. If you want to send out thousands of emails an hour, then Microsoft Office 365 is not the service for you.
The only restriction that we can see that may cause issues is the total size of an email as we have clients who regularly send larger files than the 25Mb limit, but we have alternative options for this requirement that are both inexpensive and reliable.
Basic Limits with a Microsoft 365 Account
Max Mailbox size for a single account is 50Gb
Size of a single e-mail message. The total size is 25Mb
Number of file attachments allowed in an e-mail message is 125
Number of text characters allowed in the subject line of an e-mail message is 255
Number of message body parts that are allowed in a MIME multipart message is 250
Number of forwarded e-mail messages that are allowed in an e-mail message is 30
Number of message recipients allowed in the To:, Cc:, and Bcc: fields is 500
Number of e-mail messages that can be sent from a single e-mail client per minute is 30
Number of recipients for e-mail messages sent from a single mailbox in a 24 hour period is 1500
Please note that these can change at any time.