iMac won't boot into Recovery Mode

I cannot get my iMac to boot into Recovery mode. It's a 2015 running High Sierra 10.13.6. I've tried holding down Command-R, Command-Option-R, Shift-Option-Command-R, Ctrl-Opt-Shift-R, Option, repeatingly pressing Option, just about every possible combination I've found online that says it will get the computer to enter the Recovery screen. It just starts up like normal. I've tried all of these combinations after both shutting down and restarting, with wifi turned on and off, with it hardwired directly into an internet connection, and completely disconnected from the internet. I've tried unplugging it overnight and trying everything again in the morning...nothing will entice it to enter Recovery mode.

Does anyone have ANY other way to get into Recovery mode?

The reason I'm trying to do this, is because the company I work for changed their email service, and ever since that, my Mac's email isn't working correctly. I've tried everything to try to get it to work, but finally gave up and decided to just start from scratch. Everything else on it is operating perfectly.

Thank you for any help you can offer!