Email address verification is a process by which email address validity is checked before use or in real time when validating a webform entry.
Email address validity verification is done without really sending messages to recipients. Technical queries are done at email servers level.
Email address verification is made to prevent a bad sender reputation and deliverability issues when using new email addresses or new lists. Indeed, if a new address list is of dubious quality and not checked before the first sending, ISP and webmails will note numerous errors and bounces and will give a bad IP or domain reputation to the sender.
Verification services validate easily the right part (domain part) of addresses by checking if the domain exists and is configured to accept emails. Left part (user name) is more difficult to verify because a server response is needed and some smtp servers are configured for no response. ISP and webmail services do not like email verification processes.
Therefore, email address verification is not totally reliable and verification services have not always transparent practises.
Email verification services use proxy servers to prevent ISP to block their IP address when checking.
Verification services also check (and eventually correct, flag or delete) email syntax, domain syntax and toxic email adresses.
An example of points of control with Freshadress services:
Pre-campaign validation differs from bounces management.