Software

You have not told us the reason they asked you to upgrade to the business service. If you are in fact running a business of some sort then that would be a good explanation as to why they suggested this. However if you are not then you have no reason to upgrade. They should not be blocking your email ports. If they refuse to unblock them then switch to a different provider.

Many providers do this to prevent the spread of viruses. It's a good thing

No way around it. Either upgrade or find a provider that doesn't block them.

Many providers do this to prevent the spread of viruses. It's a good thing

No way around it. Either upgrade or find a provider that doesn't block them.

They are doing this to help their users who don't host their own email or web sites. To their definition, if you are doing those things, then you are a business, because most individual, non-business users don't run those kinds of services. They are also probably doing it because they are losing money to people like you who are using a business-type level of services but only paying for individual-level connection time. You can either accept this and upgrade to their business level (which I assume probably costs more), or find another service provider. However, I have to tell you that if you're running your own email and web servers, then I assume you are using one more more static IPs, and if that's the case, then every ISP I know of will only assign static IPs to business customers.

Maybe you could find out which ports other than 80 are open then change the config of your server to use those other ports instead. You'd have to inform you clients to configure their mail clients to use those other ports.