You must have separate smtp servers for each account, and set each account to send on the smtp that has the same User Name as the sending account.

https://support.mozilla.org/en-US/questions/1350593

Thanks for the reply. I have looked at the thread: For this step...(Select account 1 in the left pane of Account Settings, then click the Outgoing Server (SMTP) drop-down so smtp1 is selected. Select account 2 in the left pane of Account Settings, then click the Outgoing Server (SMTP) drop-down so smtp2 is selected. Same for 3 and 4)...the drop down only shows Charter Communications. It doesn't show smtp1. Also this Outgoing Server option is only available when I click on the primary email account. It's not shown on the 2nd & 3rd.

The issue seems to be that you've configured a single outgoing (SMTP) server to be used by the 3 accounts when what you really need is different SMTP server settings for each.

What you appear to be missing is that you may create as many outgoing server settings as you wish, independently of your other mail account settings, by choosing Outgoing Server (SMTP) at the bottom of the list of mail accounts in the account settings, then adding as many server settings there as needed, naming them however you wish to better distinguish them. You can then associate each of those SMTP servers with a different account / identity.

Thanks. I do understand that now. But what I don't understand is how to separate them. Where would I see/add the smtp1, smtp2 and so on? Would it be better to delete the 2nd & 3rd accounts and start over? I don't remember seeing an option to add additional servers when I added those accounts.

The Outgoing Server (SMTP) settings window has buttons to the right of the list of SMTP servers to add, edit and remove server settings to that list. Is that what you're missing now or do you mean something else?

Somehow I have the feeling you didn't fully understand what I said in my first post and are still looking at the SMTP server settings from your other account settings. Look at the Outgoing Server (SMTP) settings window shown in the following article. That's what you have to use to add more SMTP server settings:

https://support.mozilla.org/kb/cannot-send-messages

That is very helpful. But this is what mine shows: Outgoing Server (SMTP): Charter Communications - twc.mail.com. Spectrum confirmed that this was correct but in the examples in the thread, they actually have "smpt" as part of the server name. Also, mine does not show Add, Remove, or Set Default...only edit.

Sorry, I have now found ADD. It currently reads as: mail.twc.com. Should it be smpt.twc.com? And then smpt2.twc.com?

Having "smtp" as part of the server name or not is irrelevant, they may name their servers however they wish, use what they told you to use for the server name.

I really appreciate your help & I promise I'm not a complete idiot but this is all like a foreign language to me. If I need to set them up as different servers, don't I need something to differentiate them from each other?

Yes, but you have other fields for that, namely "Description", where you may type whatever you wish or leave empty if you want, and "Username", which is the whole reason you're doing this, so it can be different for each despite the three accounts using the same outgoing server, and is enough to differentiate them if you don't want to use "Description" for that.

The server name must be the same for all because they're all using the same server, it's the username each account uses to access it that's different for each.