Mac OS Mail - Cannot send the message using the server iCloud

You can use this link Mail – Official Apple Support to find your mail settings .

It depends which mail services you are using , for gmail accounts that are configured with iCloud account , you don't need to add settings they are by default .

You mentioned -

Under the "Edit SMTP Server List" option my internet service providers email account is in there, do I need to add another for iCloud?

Is your iCloud mail address added in internet accounts if not add it .

See Use your Internet accounts on Mac - Apple SupportUse your Internet accounts on Mac – Apple Support (AU)

Change Internet Accounts preferences on Mac - Apple Supportsupport.apple.com › en-in › guide › mac-help › mac

Add an internet account - Apple Supportsupport.apple.com › guide › mac-help › mac

See the articles Verify an email account’s outgoing server in Mail on Mac - Apple Support

Change Outgoing server settings in Mail on Mac - Apple Support

Keep the settings as empty .

If your iCloud account is not connected ( just an example ) , open mail application click on I cloud and send a mail from - iCloud address ( in the To - field type your gmail address ) the mail will be sent to your inbox in your apple mail application a pop up appears , in this screen shot it is showing could not connect to outgoing server , thats the reason when connection doctor is opened SMTP for iCloud is totally vanished .You can apply same with gmail say if IMAP or SMTP is not working . Simply click on always use this server to get joined .

And you will observe all IMAP / SMTP for gmail account and iCloud account are showing green dots and joined to their respective server .

Compose a mail ( use command and N keys ) , fill up the credentials select a different mail address and click on try with selected server , in fact in mail preferences send new messages is automatic , that's the reason see the article Change Composing preferences in Mail on Mac - Apple Support (IE)

Send new messages from .

The email address to use when you send messages.

If you use several email accounts, you can have Mail automatically select the best address, based on the email address of the first recipient in your message, as well as the currently selected mailbox and message.

You can click on drop down arrow and choose the desired mail address like example1@icloud.com

This makes no sense. Connection Dr is no help as the one that is listed as an email address just spins all day. I cannot find anywhere in preferences that that email address is set up as a server. iCloud account is. What it means that it is sending from *******@icloud.com is beyond me. That is one email address I use but both email addresses are from the iCloud.com server. I can send emails bit having to respond to this MSG every time is irritating.

[Edited by Moderator]

Thanks for your attempt to help. I think I finally found the problem. Under Mail Server outgoing mail settings got changed from one to the other. How do these things happen?