When Smart Email Verifier communicates with the E-mail Server to determine if an e-mail address exists or not, the protocol makes it necessary to send a From e-mail address. The E-mail Server uses this information to authenticate the source and can then choose to accept or reject the communication. Many E-mail Servers use the reverse-DNS information provided by the ISP along with the IP address to authenticate the source of the e-mail. If found incorrect, the E-mail Server can choose to reject the communication.

If you are doing domain level verification then you can use any IP address. For SMTP level verification, Smart Email Verifier uses the VRFY command to verify the e-mail address. The downside to using the VRFY command is that most E-mail Servers block the command. In this case, for accurate results, you should use an IP address that E-mail Servers can use to correctly reverse-verify the source.