If you know the login credentials are fine but you still get this error, then you should:
1) go into Backend -> Miscellaneus -> PHP Info and search for “Registered Stream Socket Transports” and make sure it contains at least “tls, tlsv1.0, tlsv1.1, tlsv1.2”.
As an example, in my current install, i have this: “tcp, udp, unix, udg, ssl, sslv3, sslv2, tls, tlsv1.0, tlsv1.1, tlsv1.2”.
2) Make sure your password does not contain special chars like the & sign. If it does, change it so that it doesn’t.
3) If you are using CPanel, go into WHM > Tweak Settings > Allow all users to make external SMTP connections.
4) If you are using CPanel/WHM and you’re using a ConfigServer Firewall, there is a setting in the Firewall Settings called “SMTP_BLOCK” which says:
“Block outgoing SMTP except for root, exim and mailman (forces scripts/users
to use the exim/sendmail binary instead of sockets access). This replaces the
protection as WHM > Tweak Settings > SMTP Tweaks”
Now, if the SMTP_BLOCK is Enabled then you need to scroll down to where it says “SMTP_ALLOWUSER” and then type in the username of the account that MailWizz is hosted on. Adding the username and saving/restarting the firewall then then allow the account to connecting to external SMTP servers.