Are you the end user or sysadmin of smartermail?

The most common cause of this type of error is having the SSL/TLS settings, port number, or hostname settings incorrect. It's also possible your computer doesn't support the right versions of TLS and may be rejecting connectivity because of that. Do other email clients fail with the same settings?

In Outlook, you need to check the box for "my outgoing server requires authentication", with the sub-option to "use the same credentials as for incoming".  This is my most common mistake.

Ports:   If you use SSL/TLS encryption option in Outlook, the matching SmarterMail port needs to be configured for "SSL", which means mandatory encryption.   This is best.  

If you use the STARTTLS option in Outlook, the matching SmarterMail port needs to be configured for "TLS", which means optional encryption.

Thank you everyone for your answer and support,  I still can't add the account to outlook, my windows server is fully updated, Smartermail is well configured, i have the same configuration for all users and the problem only occurs with email users who use OS Windows Outlook, all OS Macos Outlook users work without problem. 

In Windows, I updated the Framework to version 4.8 and reviewed the firewall configuration. I also tried to configure different versions of Outlook and I still have not been able to configure the email for Windows users. I dont know what more do. 

Can I have you try to ping the SMTP address to get it's IP address and then see if you can add the SMTP server using it's IP instead? That way we can at least eliminate DNS as a potential cause. 

Hey Marlon, Just for clarity, 

1) Are you using Windows Server or Linux ?

2) Your Server with SmarterMail, is inside your local network ? or are you going out to the internet to get to it ? So all of your computers, windows and mac and the SmarterMail server are all on the same internal network ?

3) Are all of the windows computers using the same version of windows ? If so, what are you running ? 10 ? 11 ?

4) What version of outlook ? and this is Desktop outlook actually installed on the PC ?

5) Is everything wired or WiFi ?  Are the mac WiFi and the PC's wired ?