Go to Menu > Tools > Accounts and click on the General tab for your account. Scroll down to Services and tick SMTP.
Sometimes when the account settings are imported from another email application, the SMTP may not be there. The best solution is to remove that account from eM Client, then add it again using the Automatic setup by clicking the + icon on the Accounts section.
Always, before deleting an account, make a backup using Menu > File > Backup.