cant send emails but i can receive them

Go to Settings>Passwords & Accounts>Your e-mail Account>Account>Outgoing Mail Server>SMTP>Primary Server>Outgoing Mail Server>Password. Make sure that your password is entered and correct in that setting.

Then delete the account and add it back again.

My next suggestion would be to contact your email provider and see if they can help. I don’t know what else that you can try on your own, if those things didn’t help.