Go to Menu > Tools > Accounts and see if there is an SMTP tab for your account.
If not, on the General tab, is the SMTP service unticked?
If it is, tick it and try again.
If there is no SMTP tab, or option in Services, it may be because you imported the account from another email application. If one of the tabs is IMAP as in the screenshot above, remove that account from eM Client by clicking on the trash icon, then add it again by clicking on the + icon.
Thank you my problem has been solved.