Mac Mail Won't Send: "Cannot Send Message Using the Server" Fix

You try to send an email in the Mail app on your Mac and get “cannot send message using the server,” or the message just sits in the Outbox and never goes out.

Why: Sending uses your account’s outgoing (SMTP) server. If its host name, port, password, or authentication is wrong — or the server is briefly offline — Mail can receive mail fine but can’t send. Fixing the outgoing server settings clears it.

Fix 1: Check the outgoing server with Connection Doctor

This tells you exactly which server is failing before you change anything.

1. In Mail, choose Window > Connection Doctor.
2. Look at the list of accounts. A red dot next to an outgoing (SMTP) server means that connection is failing.
3. Read the message in the Details column — it usually names the problem (wrong password, server not responding, or a blocked port).

Keep this window open while you make the changes below, then watch the dot turn green.

Fix 2: Verify and fix the outgoing (SMTP) server settings

This is the fix that resolves most “cannot send” errors.

1. Choose Mail > Settings, then click Accounts.
2. Select the account that can’t send, then click Server Settings.
3. Click the Outgoing Mail Account pop-up menu, then choose Edit SMTP Server List.
4. Check the Host Name, User Name, and Password against the exact details your email provider lists (look for “SMTP / outgoing server settings” on their site).
5. Leave Automatically manage connection settings turned on if you can — Mail then picks the right port and authentication for you.
6. If your provider gave you specific values, turn that option off and enter the Port and authentication method they specify.
7. Click OK to save, then send yourself a test message to confirm.

Fix 3: If a firewall or VPN is blocking the connection

If Connection Doctor says the server isn’t responding even though the settings are right:

1. Make sure your Mac is online and try loading any website in a browser.
2. If you use a VPN, turn it off briefly and try sending again — some VPNs block outgoing mail ports.
3. Confirm that firewall software isn’t blocking email traffic on the standard outgoing ports 25, 465, or 587.

FAQ

My message is stuck in the Outbox — is it lost? No. It stays in the Outbox and sends automatically once the outgoing server connects. Fixing the SMTP settings above releases it.

Receiving works but sending doesn’t. Why? Receiving and sending use different servers. A working inbox only proves the incoming server is fine — the outgoing (SMTP) server is configured separately, and that’s what to check here.

Where do I find my provider’s SMTP settings? On your email provider’s support site, under “IMAP/POP and SMTP settings” or “outgoing server.” Enter those exact values in Edit SMTP Server List.

Sources: Apple Support — If you can’t send or receive email on Mac, Apple Support — Verify an email account’s outgoing server in Mail on Mac, Apple Support — Change Outgoing server settings in Mail on Mac