Sending mail with alternate "reply-to" address

For years I've been able to send mail (Apple Mail app) from my main address ("Rick@..." at my own domain) or choose a "Support @..." variation, but it suddenly stopped sending the emails just a few days ago, without my changing any settings or OS updates. Incoming Support mail still appears, since the server redirects it to the personal mailbox. Mail appears to send the Support emails, but they cannot be found to be displayed -- and I suspect they are not actually being sent. Clicking the little Reply icon does not display the reply I composed, only an alert "Error. Mail was unable to find your reply to the message. You may have deleted the message."

My main account is set up in Apple Mail as POP. Also in Mail, I have had a second account set up as IMAP (otherwise it won't set it up because it sees it as a duplicate of the personal POP version) that sends using the same settings as the main account, the only difference being the "Support@" Reply-to field. It's important to be able to choose my personal address or the Support address when I compose a new email.

Something that now appears different is, under the "All Sent" mailboxes, is that Support's "Sent" mail no longer has the paper airplane icon, but has a blue icon with @ symbol, and under that is the Inbox of my main account.

Maybe this method simply no longer works.

Is it still possible to send using an alternate Reply-to address and have both regular and alternate addresses appear in the From popup menu when composing a new email?

Thank you in advance for any suggestions!