what is the latest iOS available for Mac book pro ?

I don't think that iOS is available for any MacBook Pro model. The Mac computers run macOS, while iPhones and iPads run iOS. The latest Apple silicon M1 computers can run iOS apps, but that's all.

If you meant to ask what version of macOS is the latest available for your particular MacBook Pro, then first of all, you have to mention which model it is. If you click on the Apple logo in the top left of the screen, then on About this Mac a window will be displayed with lots of information. The first line shows your current macOS name, the second line shows the exact version number, while the third line shows you exactly what model of Mac you're using.

I see Apple recommended a different post from the first response posted but the one I made had all the information you needed (which the recommended article did not, so ???). I too noted that what you need is macOS and not iOS. I also told you how you could determine which model Mac you have and then how to use this information to find various available system upgrades.

None. Macs don't run iOS, they run macOS.

Upgrading a macOS depends upon the model Mac, specifications, and year. To get more information about your computer, choose:  (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in "About System Information [Profiler] on your Mac" - https://support.apple.com/HT203001

Use the computer information, including the model year, and the links below to determine what is possible with your computer.

Click on this link to read how to get different versions of macOS - https://support.apple.com/HT211683

- The article also has links to system requirements for each OS version which you should check against your exact computer model. Note some of the model identifier articles are outdated. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

- Make sure you have more RAM (memory) than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get links for High Sierra and later using the Mac App Store application to open a functioning page.

- Sierra, El Capitan, and Yosemite download as a .pkg. Click on this to have a converted version placed in your Applications folder as an "Install OS..." application. Run the installer and follow the directions.

- Upgrades may take some time (up to an hour or more) to install, with restarts and blank screens.

Before upgrading, back up your data in case something goes wrong. See this support document: Back up your Mac with Time Machine - https://support.apple.com/HT201250 or use a cloning tool.

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/

In that case you can run the LATEST version of macOS, which is Big Sur. You can find it on the Apple Appstore. Please back up all your data first, though.

As for Hotmail, that became part of Outlook.com years ago. I have a Hotmail address, but I just read it using Safari or Google Chrome on my Mac. I'm not sure why there would be a problem adding it to Mail. You can try under Add Account, then select either Microsoft Exchange or Other Account.

Until recently my wife was running Hotmail in Mavericks Mail and now uses it with El Capitan Mail so it isn't a matter of you needing a higher Mac OS. What Hotmail likes doing is changing mail servers and this has to be hand-edited in Mail. I don't remeber eactly what we did with ours but I did a general web search for the error and "Mac Mail" and found others who reported what they did.