When an email does not leave

Could not authenticate

The server refused username or password. Check both; for Gmail and Microsoft 365 you need an app password, and Microsoft 365 must allow SMTP authentication for that mailbox.

Could not connect / timed out

The site cannot reach the server. Check server name and port. Some hosts block outgoing SMTP ports: ask your hosting, or try 465 with SSL or 587 with TLS.

Certificate or TLS errors

The encryption must match the port (TLS with 587, SSL with 465), and the server name must be the one in its certificate: use smtp.example.com as your provider writes it, not an IP address.

Sender refused (550, 553)

The server does not accept that From address. Use an address the SMTP account is allowed to send from, usually the account itself or one of its aliases.

Sent, but in spam

Ask your provider for the SPF, DKIM and DMARC records of your domain and add them to the DNS: they are what tells the recipient's provider that your server may send for your domain.