I need to use an account only to send emails, but I don’t want that account to be synchronized as a receiving account
Look at the Account page Menu/ Tools / Accounts
On the bottom of page is at least 2 check boxes
SMTP (send) IMAP or POP (to receive)
uncheck the one you do not want
Richard Morino, Thank you very much, but I’m sorry to inform you that it only works if the account is not the default one.
Yes the default account does need to have in incoming protocol. There is no way around that unfortunately, except to put in an incorrect password for the POP/IMAP protocol. But you will need to turn off showing a window when an error occurs, because you will get plenty.
Gary Curtin, Thank you very much. Unfortunately, the problem is much deeper. When “STMP” is unchecked, you can not use this account to send and receive emails anymore. It also doesn’t work when “STMP” is configured using an incorrect password.
I think you misunderstood. You need to have SMTP checked. That is the protocol to send the messages.
If you want to make the send only account the default, it needs to have the POP/IMAP and the SMTP protocols otherwise it cannot be set to default. You already know this. But because you don’t want to use that account for receiving email, the receiving protocol (POP/IMAP) needs to be disabled somehow. You could just give it an incorrect password. That is incorrect password for POP/IMAP, correct password for SMTP.
Make sense?
If your intention is to trash any in bound mail to this special account then use a Rule that is attached to just this account to just trash all inbound mail. For a none default account un checking IMAP protocol should prevent any email from coming in. Just have the SMTP checked for this none default account. One can also only allow specific email address be allowed to come in if that is your desire using a Rule. I created a rule for one of my accounts to delete if spam and it does not even go to the spam folder. it is just gone.
If you only have one account in emclient then a rule seems to be your only option.
Great idea, Richard. Rules are a much better solution. 
Mine requires a bogus password, which means errors in receiving, which means turning off the error window, which can mean a whole lot of problems later on.