Hi @mattymil ,
This does sounds like a drag, While I haven’t personally used Microsoft I have not heard such an issue with this little amount of sequence going out.
If your email and domain is very new then, this could cause an issue because Microsoft is fairly strict with their email spam policy.
Have you considered warming-up your email (i.e using a service like warmbox) before you start sending out the emails?
The issue you are facing is very specific to your account so I would try to get to HubSpot call support(because you have pro-Hub, why not use that) and see what they recommend.
best