mail makes noise that it has sent an email but leaves it in draft box

My Apple Mail periodically makes a swoosh noise for sent mail from different email accounts but does not send the email, but instead moves it to Drafts. I now have to check that all emails are sent and often need to resend them if they are stuck in Drafts.

Additionally, Apple Mail sometimes asks to confirm the server. Once I choose the server and try to push send, the SEND button does not do anything. I have to close the email which sends it to drafts. If I open the eamil from Drafts and press send, the email is sent.

Help! Any ideas welcome!