Roundcube - web-based mail client
Outlook - application - the same client
Thunderbird - application - the same client.
So for connect to your mailbox from another client (not RC) you must know your imap and smtp server settings, your login and password.
As I see you not IT-engineer, so we will try find this settings for you. For example you have mail like: user@domain.com
Open this link (
https://mxtoolbox.com) enter your "domain.com" (second part of your email after sign @) and press "MX Lookup" - after 3-5 sec you will see table with Pref, Hostname, IP Address,TTL.
I think you can see your server Hostname.
And next step - setup any imap client (outlook, thunderbird, iOS client, gmail and etc) with this settings (need test different variants for IMAP and SMTP port)
Imap server: your Hostname from link
Imap port: 143 or 993 (with SSL/TLS)
smtp server your Hostname from link
smtp server port: 25 or 465(with SSL/TLS) or 587 (with SSL/TLS)
user: user@domain.com
password: your password.
I think this must be successful case.