TLS 1.2 in Apple Mail 9.3

Apple updates their Mail client as part of the operating system upgrades. That is just the way Apple behaves. The volunteers here are just trying to figure out if it is possible for you to upgrade your Mac.

You are not required to provide useful information, and no one wants your serial number, although some websites use the serial number to decode the manufacturing date of the Mac, but there are other ways to figure out what Mac you are using and if it can run a newer version of macOS.

If you do not wish to upgrade to a newer version of macOS, then look at getting a 3rd party email client that will provide TLS 1.2 on your El Capitan install. Thunderbird is free <https://www.thunderbird.net/en-US/thunderbird/68.1.2/system-requirements/>

There are other email clients out there.

I realize this is not the answer you want, but it is the best I can come up with.

Without proper identification of your apple product for your information

an idea would be to check to see what can be done, because some have

limited upgrades past El Capitian 10.11.6, still.

Just that 'latest versions' may be shown which could be beyond an older Mac.

• Identify your MacBook Pro model - Apple Support

https://support.apple.com/en-us/HT201300

Further above El Capitan, is "Sierra macOS 10.12.x" info, + links are shown.

And separately further long, is "High Sierra macOS 10.13.x" (blue=link.)

These if supported by the hardware build version of your Mac, may be

chosen; but some may have been obsoleted, or skipped. The fully new

'macOS Catalina' may not be advised; unless you have separate Mac.

Each of the above links has info on how to tell if whatever your Mac is,

could be upgraded past El Capitian. In any event, be sure you have a

good backup; on separate external drive. Also consider extra hardware

that could support a full system Clone; & powered external HDD, for it.

The identify page goes back before 2010; so that may help along with

those installer info pages. Issues for some users are reasons to hold off

on upgrade past High Sierra or Mojave, for your daily work machine.

Short of needing to get new printer, drivers, & maybe 64-bit software.

.. https://duckduckgo.com/?q=how+to+upgrade+apple+mail+TLS&t=ffab&ia=web ..

search results found within above:

Good luck & happy trails!🌞

Use the 'About this Mac' to see the exact build of your Mac and

then the Mac OS X that it may be upgraded to run. ~ And then

perhaps there is a reason why your Mac cannot get past 10.11.6.

If your MacBook Pro is newer than 2009 build, it can upgrade later OS.

The software you seek is an upgrade, available in the next or later

system version. So I asked once and again, which build year version

is your MacBook Pro; and sleuth the facts from your Mac to answer.

(You'd need to upgrade the system to a later version, that has more

recently supported TLS Apple Mail, & etc, to see an improvement.)

That is where a newer supported Browser (Safari) could have to

come from; unless you sought to keep El Capitan 10.11.6; such as

I have, and choose to get/use Mozilla Firefox Quantum 69.0.3.

And a browser can supplement the lack of Apple Mail, if you use

'web-mail' instead of built-in Apple Mail. ~ So that's two answers.

(I've also written many things that would've been edited by automatic

**** IF I'd written @#$%&% instead of nice words; but I removed them.)

If you cannot identify your Mac correctly and with the details necessary

to lift your credibility a bit higher; then shame is not on me for trying.

..waiting to see who's mad enough next to put their neck on the block here..