Work through these in order before opening a ticket.
1. THE USERNAME
It is the full email address, not the part before the @. This is the most common cause by a wide margin.
2. THE PASSWORD
Reset it in cPanel > Email Accounts and enter it again carefully. Autofill frequently supplies an old one.
3. THE PORTS
Incoming 993 with SSL. Outgoing 465 with SSL, or 587 with STARTTLS. Some networks block one; try the other.
4. OUTGOING AUTHENTICATION
It must be enabled, with the same username and password as incoming. Mail that receives but will not send is almost always this.
5. THE HOSTNAME
mail.yourdomain.com works once the domain resolves to us and AutoSSL has run. Before that, use the server hostname from your welcome email.
6. THE FIREWALL
Repeated failed logins block your address. If nothing connects from your network but works on mobile data, that is it. Open a ticket with your address from whatismyip.com.
STILL STUCK
Open a ticket with the client, the exact error text, and which of the above you checked.
