I want to import my inbox, sent and contacts from opera mail to em-client. Can you please help me out.
From what I remember, you can’t do that directly.
If your account is setup as IMAP, then the messages are already on the server, so setting up the same account in eM Client will sync the messages. If you were using POP3, you can setup an IMAP account in Opera Mail and copy the messages from the POP3 folders to the IMAP folders.
There is another way to import messages. You can export Opera Mail folders as mbox files. Then you can import them into eM Client. Just change the extension from .mbs to .mbox.
For contacts there is a handy utility at SourceForge to convert the Opera Contacts file to csv.
https://sourceforge.net/projects/opera2gmailcsv/which you can then import into eM Client.