Error sending bulk emails

I am sending emails internally to employees based on a spreadsheet. I might send around 300 emails at one time (each email containing an attachment sent to a specific staff member). I am using the SMTP mail activity using our Office 365 server and port 25. There are many times where a certain number of emails get sent and then the robot fails. The error I am getting is:

Unable to read data from the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

Any ideas what might be causing this? We checked firewall rules and that does not seem to be the issue.

Just a thought - any specific limits that might be set on Exchange or O365 side? Like emails per hour, etc.?

Supposedly not, but I am in touch with Microsoft to see if it on their end. The emails do not get rejected, the robot just stops and we get the error.