SMTP error messages are often more literal than they sound. Here’s what the most common ones mean and how to resolve them.
“Relay Access Denied”
This means the mail server refused to send a message on behalf of an address it doesn’t recognize as authorized — almost always because the sending device or application isn’t authenticating correctly. Fixes, in order of likelihood:
- Confirm your email client is set to use SMTP authentication with your full email address and current password, not just the username.
- Double-check the outgoing (SMTP) server address matches your host exactly (see cPanel’s Connect Devices tool for the exact settings for your mailbox).
- Confirm you’re using the correct port (587 with STARTTLS, or 465 with SSL) — port 25 is frequently blocked by ISPs for outgoing mail and shouldn’t be used from a desktop client.
“Mailbox unavailable” / 550 errors
A 550 response usually means the recipient’s server rejected the message outright — commonly because the recipient address doesn’t exist, or because your domain failed an SPF/DKIM/DMARC check on their end (see our authentication guide) or is listed on a spam blocklist.
“Connection timed out”
Usually a network-level issue rather than a mail server one — check that your local firewall or antivirus isn’t blocking the SMTP port, and confirm you’re not on a network (some corporate or public Wi-Fi) that blocks outbound mail ports entirely.
Authentication failed (535)
Almost always a wrong or expired password saved in the mail client. Re-enter the current mailbox password, and confirm there isn’t a typo in the username (it must be the full email address, not just the part before the @).
When to open a support ticket
If settings are confirmed correct and errors persist, check whether your server IP appears on a public blocklist (search “MXToolbox blacklist check”) — this is something our support team can also help resolve directly if a delisting request is needed.