How can I stop opened Outlook emails downloading to iPhone/iPad

Hello, I've been using Outlook on my computer to read emails for some years. Previously, any emails that came through to my computer weren't subsequently sent to my iPad and iPhone. (If I received emails on my ipad and iPhone first, then they would still download to my computer afterwards, which was all good!)

I now have a new computer and I find emails are being sent to all 3 devices, regardless of whether I have received them in Outlook first. Is there a setting somewhere to stop this? (I'm spending ages deleting emails from that iPad and iPhone that I've already seen!)

I'm not using iCould, by the way!

Many thanks for any suggestions anyone may have!