The use of VERP requires each message to be sent once for every recipient, instead of once to each receiving SMTP server. This is because of a limitation of SMTP, which allows multiple recipient addresses to be specified in a single transaction, but only one sender address. When there are many subscribers in the same domain, a mailing list that is not using VERP can combine multiple deliveries into a single transaction. It connects to the appropriate server for the domain, gives the single sender address, the recipient addresses, and then sends the message contents only once.