When an iMIS communication suite email ends up in the junk folder, it is important to take the following into account:
1) Is there an SPF Record created for your iMIS App server's sending IP Address when sending from your domain's (FROM email field) email address in the communication template?
2) Are you using the same sender's domain's email address that contains the SPF Record?
3) If you are using more than one company domain email address in the (FROM Field) of your communication template, as an example (email@example.com, or firstname.lastname@example.org), and only domain1.com has an SPF record, it is very likely these emails sent using the other domain2 in the from field (email@example.com) may end up in the junk email folder, or being caught by the anti-spam filtering solution.
In order to mitigate these type of case scenario, please reference to the following article:
Similarly important, if the recipient is using Office 365 as an example, you should recommend them to review, and follow all the best practice suggestions by Microsoft or their main company's mail server provider.
Also, If emails are going out from a contact's profile page (email button), that uses the communication suite and that uses the SMTP server. If some users are not getting the emails from iMIS but others are that would point to an issue on the user's side. Either the email is getting sent to their SPAM or junk mail folder or their email server is blocking the email for some reason.
Similarly, Emails could end up as (False positives, or False negatives) in the anti-spam filtering solutions. In those case scenarios, it appears to be going into the junk folder. As a result, it is also advisced to review and follow all the recommendations as outlined by Microsoft Office 365, and that should help mitigate this type of concerns:
How to prevent real email from being marked as spam in Office 365
What is an SPF Record:
A Sender Policy Framework (SPF) record is a type of Domain Name System (DNS) record that can help to prevent email address forgery. ... Adding an SPF record can help prevent others from spoofing your domain. You can specify which mail servers are permitted to send email on behalf of your domain.
Other Suggested Public KB Article for Review: