What macOS mail applications support a "Send Only" email account?

Question

I'd like to revisit a question which has come up before (more on that below), but which does not, in my opinion, have a good “future-proof” solution looking forward to 2019 and beyond.

“Send Only” Email Accounts

For years I have used Mailsmith as a “send only” email app, and it works very well. The primary reason for this is that it understands the concept of a “send only” account -- an account which is specifically set-up in such a way that the app will be used to send but not receive email.

The Problem

The problem is that Mailsmith is 32-bit, and according to its own homepage “will not be upgraded for 64-bit macOS compatibility.” That means it will throw a warning in Mojave when first launched (no big deal) but will probably cease to work in the next version of OS X (big deal).

What I’m Looking For

• Because I can register Mailsmith as my default mail application, it will handle mailto: links on web pages, or any actions which want to send mail through a regular Mac email app.

• I can setup multiple accounts in Mailsmith (work, personal, etc) and have the correct “From:” address be used when the email is sent.

• Mailsmith integrates with my Contacts.app address book, so it will auto-complete email addresses as I type them.

• Mailsmith can save Drafts on the SMTP server, and properly adds sent emails to the “Sent” folder on the SMTP server.

Previous Suggestions.

As I mentioned, this question has come up before, but none of the proposed solutions address the above points.

For example:

1. Setup another mail account and delete all email sent to it. This answer probably comes the closest, but won't put sent messages in the proper “Sent” folder on the server, and it may not use the proper “From” address either. I have found that to be frustrating when I need to go back and refer to previous mail threads.

2. QuickMailer: The app hasn't been updated since 2014, and doesn't seem to support mailto: or multiple accounts.

3. AppleScript: This won't integrate with Contacts.app address book, and risks losing the email if something interrupts the process after starting to write it. Seems far too fragile.

4. QuickSilver: The app is pretty much dead, and most of the plugins haven’t been updated in years.

5. “Let.ter”: The app (which I owned and had high hopes for) is not only buggy, but is now abandoned and hasn’t been updated in years.

Other Native Mac Mail Applications?

I use MailMate as my regular mail application, and have tried replacing Mailsmith with Apple’s Mail.app and Mozilla’s Thunderbird. However, neither app seems to accept leaving the incoming mail server information blank. That means they will try to download my email if I enter the correct information, and if I enter incorrect information, they will continually throw error messages.

Are there other Mac email clients that I should try? It seems like most of the mail apps that I’ve heard about recently are for iOS, not Mac, but it’s entirely possible there are others out there that I have overlooked.

{I will try to update this post as I find and try other mail clients.}