Outgoing mail server gone under El Capitan

Immediately after installing El Capitan, I am unable to send emails. However, I'm still receiving them. Note the following oddity ... although I once had an email address "drastal@comcast.net" it has been inactive for years. Nonetheless, it appears when I view the "Internet Accounts" tab under "System Preferences". I've asked staff at the Apple Store to resolve this twice before, and no one seems to understand why it's shown there. Yet my drastal@icloud.com account has worked perfectly, until I installed El Cap.

The contents of my Mail Preferences are shown below. Thanks in advance ...

iMac (27-inch Mid 2010), OS X El Capitan (10.11)

Posted on Oct 1, 2015 4:02 PM

Aug 14, 2016 6:14 PM in response to Drasticbunny

I have this question too.

Some differences however.

1. My System Prefs intermittently(?) fails indicating the preferences panes will not open. And some pref's panes do and some do not open. iCloud, Internet Accounts, and Network usually fail to open when this happens. When this happens, however, I can still get to my iCloud via Browser and AppleID Password.

2. Opening ANYTHING takes 12 to 15 seconds after keypress or pointer click.

3. Almost all applications from the dock do not open, instead an "Application not Responding' appears at the top of the clicked application Dock ICON.

4. You can hear constant Disk activity.

5. On attempting one of your respondents' answers to 'turn off Mail' in the iCloud Sys Prefs's window: a) it failed to turn off at first, then it would not turn on again. I had to close Sys Prefs, run iCloud in the browser (elapsed time o.o.a. 4.5 Mins) and re-run Sys Prefs being careful to not click anything more then once (i.e. wait it out).

6. Most of the above is the result of starting and stopping WiFi and Ethernet ports and something in Apple OS X running called "symptomsd" of which I can find neither reason nor record. Guessing it has something to do with crash reporting.

7. After re-enabling iCloud Mail in the Sys Prefs., closing prefs and reopening to check all's well and closing again. I return to Mail Client.

8. Getting error still that reads:

This message could not be sent. It will remain in your Outbox until it can be sent. This message could not be sent because your account does not have a preferred outgoing mail server. Select an outgoing mail server from the list below."

9. I find no outgoing mail servers and no place to compare "preferred" outgoing IMAP mail servers to any other. Raises the unanswered question: Is this related only to the ISP involved? To iCloud servicing of other ISP systems? to another (Global) outgoing IMAP preferred (or 'Default") server in Mail.app? Does "account" in error message refer to ICloud, Gmail, AOL, Yahoo mails? What about mail clients in browsers, i.e. FireFox, Safari etc.?

What does my client software need to send this mail???