Problem with outlook

[thesheep20]

Hi, I'm having a very strange problem with outlook. I help run a very small business and recently all the e-mails we send out from outlook have been going into people's spam folder. While checking around, we've noticed that emails sent from the web portal for email from the site that hosts our website don't go into spam. I've noticed as well that it seems to happen with Gmail recipients more than others. I've tried a few things suggested like removing signatures etc. but nothing has worked. Wondering if you guys have any advice. While using the web host email is a temporary fix, it is much more convenient to use outlook. We use the Outlook app that comes with windows. Any help would be much appreciated, as I'm much more comfortable with hardware than software.