Sajjad Zaidi is detailing that Dancom has started blocking port 25 on its network. Port 25 is used for operating email servers. This means that ordinary users of the service would not be able to operate mail servers on their own. Of course, Dancom provided SMTP server would be there to accept mails for onward sending.
Basically this is what every major ISP eventually concludes as the right thing to do.
By doing so, and doing so collectively, Pakistani ISPs can contribute to the global Internet community by ensuring that small sources of spam (that can collectively be a huge problem) are stopped from spitting the junk to the Internet at large. The ‘cost’ involved in taking this step is obviously the efforts such ISPs would have to put in to face the customers who would ask for this facility. The disputes, although not very likely, can also enter legal domain with questions being asked along the lines of Net Neutrality and consumer rights.
Applying the restrictions to the pre-paid card users seems to be most justified. These users have no identity-revealing-relationship with the providers other than the CLI information. For account-based customers and other SOHO/business customers, the step would be difficult to implement.
Acceptable Use Policies and Terms & Conditions of the ISPs taking this step will require updates to ensure that users are told about the fact that they cannot use the service for running their own email servers . Quality (uptime, queue holding capacities etc) of SMTP servers at the service provider end will also to be enhanced. Customer services staff that face the customers will require training and provisioning of background information on the subject matter to fully satisfy the customers who can mistake this netizen-friendly step as a draconian right-denial thing.