Problems with sending email from Windows 10 mail

[#1]

Hi, 
I have been unable to send email messages from windows 10 mail.  I am able to receive messages.  Tried to use the online guides, but they do not seem to fit with the program I am using.  Any help would be gratefully received.
Sarah.

[#2]

Hi Sarah,
What happens when you try sending?
These are the settings you'd need for sending from a Plusnet account:

[#3]

This is the message I get,
'We weren't able to send this message, so we've put it in your Drafts folder. Before you try sending it again, you can check to see if the address is correct and that no attachments are too large.'
It is just a small email to test the system.  The address is correct and not that large.  I have used the settings you have showed to set the mail program up.  I have no issues with downloading the emails from the server, so really confused.

[#4]

If you're using IMAP rather than POP3, try setting the account up again but use your full email address as the server username. You shouldn't need to do this with the latest version of Mail, but it used to be necessary with earlier versions.