Created dev O365 Tenant to test email, everyting blocked with "5.7.708 Service unavailable"

A couple of days ago, I created a development O365 tenant with my Visual Studio subscription, in order to try to implement Exchange-Online-based email notifications in my software. Everything seems to be working (I'm using the Graph API to send emails), but I always get a message in the sender's inbox similar to this:

Diagnostic information for administrators:

Generating server: SN7PR06MB9083.namprd06.prod.outlook.com

[redacted.recipient]@[redacted.tenant].onmicrosoft.com
Remote server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653

I just got this tenant, and I've never managed to send a single email successfully. Are the dev tenants just on all the block lists because people were using them to spam?

Anyone have any suggestions?