Email Delivery, Settings, Greylisting issues

I am looking for someone with a thorough understanding of mail servers, mail delivery and record settings for the following reason: We have our domain hosted with Siteground, which also hosts our website and our DNS settings. We have our mail / calendars hosted with Exabytes (see their business email offering on their website) which is actually running the Smartermail software. Since Exabytes did not give an awful lot of information on what we needed to do to have our mails running on their business email service we slowly managed to find out which DKIM setting and SPF settings we needed to create in Siteground and got to a point over a number of weeks where most of our email seemed to be working well. This then still left issues with calendar invites, which were often arriving in our clients spam / junk folders. There appears to have been an improvement since we amended our records on DNS settings, but we still find that clients who are running Microsoft Exchange and Google services often do not see all our emails or invitations. To clarify: we do not send out bulk emails at all. The non-delivery we are experiencing is with existing clients, where it might just be that a different team member (like an assistant) cannot get their email delivered to the same client that another team member is working with (mail sent from the same domain). So before we decide that Exabytes is not going to work for us and pull our entire mail and calendar functions out to host that somewhere else, it makes sense to investigate what is causing the problem. If you know that you are able to analyse, spot the issues and make the improvements to fix these delivery issues / understand how to get our domain or IP whitelisted then please indicate that in your response in enough detail so that it is clear to me that you actually understand the task at hand (there are allot of gereric responses on freelancer and I need to see from your response that it is a real option. Thank you in advance for bidding.