As Office 365 is cloud based, Microsoft controls elements such as the size of your email inbox and the maximum size of email attachments. However one interesting point that stood out in a blog post was that you could only send 30 emails per minute. This raised the question as to whether it would be possible to send an email to a large group, for example ‘All Staff’. We have not tested this on a very large scale, but we have been able to send more than the 30 emails per minute limit, we were able to send in excess of 60. We did not reach a point where we could not send anymore however. Currently we are limited to 15 users, so a large scale test is not possible. The post specifically mentioned the Enterprise version, and I would be surprised if there was such a limit in place, say 30, 50 or 100, as a large Enterprise business could have more than a thousand users that you may wish or need to email.
It turns out that there is a limit of 30 emails per minute, however if more are sent they are still delivered. They may be queued on the server to prevent overconsumption of system resources though.