Email ending up in clients Junk folder

Hi guys, I've recently discovered that our Email is ending up in the Junk folder of various recipients. I haven't altered or changed anything and this has only just started happening in the last 4 weeks or so. All Email is sent from within Homestead Email login.

Any idea what could be at fault? It has caused confusion recently with people who we Email fairly regularly, hopefully someone can point me in the right direction.  

Thanks in advance.