Siteground-webmail incoming emails landing in spam folder

I have a client who has an email address through siteground (webmail) and for some reason, some of the incoming emails from their customers seem to go directly to spam. I can’t figure out a rhyme or reason why certain emails are showing up in the inbox and others are going to spam.
Any idea how to resolve this for the future?
(I know the emails can be moved to inbox… the question is how not to have them going to spam in the 1st place…)
Batsheva Sofer

Spam filters are tricky…there are some tricks for getting emails from a website not to be spammed, but I don’t know any for this, sorry!