I don't want to see new emails in thunderbird, I just want to be able to use thunderbird for sending out emails. How can I do that? I could put in garbage imap server but then sent emails don't seem to get stored in sent foler correctly.
A Thunderbird account is only for receiving mail. It cannot send mail at all.
To send mail in Thunderbird, you use an identity, not an account. Unfortunately, Thunderbird's user interface makes the relationship between accounts and identities confusing, as the "Manage Identities" button is found within Account Settings.
Add an identity to your default account, and if necessary add a separate SMTP server for the new identity, but don't otherwise configure the account. The new identity can send mail independently of the account, and it will never try to receive mail.