Apple Mail cannot send email

Separate the mailbox from the app

Send one message from Roundcube

If Roundcube sends successfully, focus on Apple Mail's outgoing-server configuration or the local network. If Roundcube also fails, retain that exact error and stop changing the Mac settings.

Mail on Mac

Edit the SMTP server

1. Open Mail > Settings > Accounts and select the mailbox.
2. Open Server Settings.
3. Under Outgoing Mail Account, choose Edit SMTP Server List if needed.
4. Use the assigned UKC server hostname, not a raw IP or mail.your-domain.
5. Enter the complete email address as the username and the mailbox password.
6. Use port 465 with SSL / TLS, or port 587 with STARTTLS.
7. Save, quit and reopen Mail, then send one external test.

Correct outgoing details

Authenticated UKC SMTP

Server

Your assigned UKC hostname, for example ukc12.uk

Username

Your complete email address

Password

Your mailbox password

Preferred port

465 with SSL / TLS

Alternative port

587 with STARTTLS

Authentication

Password, required

If it still fails

Check the exact outcome

Password rejected

Verify it in Roundcube and update the outgoing credential separately.

Certificate warning

Use the assigned UKC hostname and check the Mac date and time.

Connection timeout

Check port and encryption, then compare from another trusted network.

Message stays in Outbox

Open Connection Doctor and record the SMTP result.

Sender rejected

Keep the complete server response; do not repeatedly resend.