SMTP diagnostics tool — test connection, TLS, auth & send

What is SMTP diagnostics?

SMTP diagnostics verifies that your outbound mail server accepts connections, advertises the capabilities you need (STARTTLS, AUTH), and can authenticate your account before you configure it in MailFleet or another sender.

Unlike a simple port ping, a full SMTP conversation walks through EHLO, TLS upgrade, AUTH, and optionally MAIL FROM / RCPT TO / DATA — surfacing the exact response codes providers return when something is misconfigured.

When to run SMTP diagnostics

Run diagnostics after creating a new SMTP profile, rotating passwords, enabling MFA with app passwords, migrating providers, or when campaigns fail with authentication or TLS errors.

Agencies should test each client SMTP endpoint before scheduling permission-based sends. A five-minute diagnostic often prevents hours of bounce analysis later.

• New SMTP or API-to-SMTP relay setup
• After DNS or firewall changes
• When port 587 vs 465 vs 25 behavior is unclear
• Before first production campaign on MailFleet desktop

Ports and encryption explained

Port 587 with STARTTLS is the modern submission standard: plain connection first, then TLS via STARTTLS, then AUTH. Port 465 uses implicit SSL/TLS from the first byte. Port 25 is traditionally relay — often blocked for residential and cloud IPs.

Match encryption mode to provider documentation. A common mistake is selecting SSL on port 587 or STARTTLS on port 465 — the verbose console shows where the handshake fails.

Security and responsible use

This tool connects from MailFleet servers to the SMTP host you specify. Credentials are used only for your session and are not stored. Image captcha, per-IP rate limits, and private-network blocking reduce automated abuse.

Send test messages only to addresses you control. MailFleet is built for permission-based email — not bulk unsolicited mail. Use test sends to validate configuration, not to blast lists.