E-mail verification involves three steps

1) Syntax verification
2) Domain verification where Smart Email Verifier communicates with a DNS server.
3) SMTP verification where Smart Email Verifier communicates with the correct SMTP server.

An e-mail is valid or invalid only after all three steps are completed successfully. Some SMTP servers don't respond to queries from an external application for security reasons. In effect, the SMTP server blocks the connection. Smart Email Verifier reports an 'SMTP Connection Blocked' problem during such times.