I'm currently working for an international company that sends over 400 emailcampaigns a year. The last month we've noticed that the delivery rate of our campaigns has dropped with 20% and some customers started complaining that they don't receive our emails anymore.

All these clients receive our emails on an emailadress hosted by our competitor. Is it possible that they are blocking our emails? And how do they do it? With our IP-adress or do they block every email that has been sent from the emailadresses hosted by my company?

The strange thing is that some people still receive our emails even though they also have an e-mailadress hosted by our competitor.

Kind regards