An Internet Service Provider (ISP) is an email receiver that provides a mailbox as a value add-on for having purchased telecom/television services like broadband or satellite TV.
E.g. @btinternet.com and @virginmedia.com.
A Mailbox Provider (MBP) is a company who provides mailboxes for free and their income is generated from advertising revenue.
E.g. @google.com, @hotmail. and @yahoo.