Good mornig, I got to set a pop3 (libero account) on em client but I need to leave messages on
web and on other device (mobile and another computer).
Can anyone tell me where can I find this setting on Em client?
thank you for your support 
When setting up your account (Menu > Tools > Accounts) choose Mail > Other.
After entering your email address, choose IMAP and enter the details of your mail server.
Once the account is setup, this will sync the server with eM Client. So you will see the same items on your computer as on your phone.
Any messages that you have already downloaded with POP3 that are not on the server, you can now move to the IMAP Inbox folder. Once that is done you can delete the POP3 account from eM Client.
Thanks for your answer Gary, I didn’t start with imap 'cause I read that with libero email is better not to change account settings from pop3 to imap once you have started.
This account is also setting on Outlook 2016 with pop3 with “leave copy of messagese on server”
Leave copy of messages on the server and IMAP are two different things.
Leave a copy does exactly what is says. Once you have downloaded the message to your email client, it is not deleted from the server (unless that is what you specify). You need to make sure that all of the email clients that access that account have the same setting, otherwise one of them might delete the messages from the server before another can get them.
If you are happy with POP3 and want to continue like that in eM Client, you can continue to use that protocol. In the POP3 settings for your account, down towards the bottom you will find that option of how to leave messages on the server.
IMAP simply syncs what is on the server with the email client. That includes other folders and Sent items. So if you create a new folder in you Inbox using your email client, it will also be visible in other email clients using IMAP. Also if you send a message, that sent message will also be seen in any other client setup as IMAP.
Your mobile is most likely setup as IMAP already.
The only problems changing from POP3 to IMAP, is that messages downloaded from the server to local POP3 folders may no longer be on the server. And sent items also may not be on the server. But that is easy to solve as I explained above. You first create the IMAP account, then move any messages across from the POP3 Inbox and Sent folders. (Sorry, I forgot to mention the Sent items.)
Hope that makes sense.