Problem with Office365 Bulk Mail?

Question

Microsoft Office

The essence of the problem is that when sending a mass mailing from a shared mailbox to addresses that are not in the tenant, this mailbox is blocked, and half of the messages are not delivered. I read that office365 has a message limit of 30 messages per minute. But then it is written that if this limit is exceeded, then messages are distributed with a large hourly interval. That is, 30 will come in one minute, and then the next 30 in another minute and does not reach the blocking of the box.
Mass mailing consists of about 50-60 recipients, this mail weighs about 60-80 kb.

Do not tell me how to avoid blocking the box? Maybe you need to set something up in protection.office.com?