It’s spam they’re concerned about. Spam email is kinda “big business” and one way they thrive is by using bots to just scan for poorly-configured or vulnerable systems to hack and install an app that will let them send email from your system. By compromising hundreds or thousands of individual machines it makes it hard for mail providers to block them individually. It also uses a ton of bandwidth on internet service providers networks.

So some time ago service providers started to simply block port 25 (used to send email) on their networks except to certain services. I think they’ve backed off a bit now but inbound port 25 can often be blocked still. It may even be against their TOS in some cases.