Sometimes emails sent to clients, either directly or through the use of an email monitoring campaign are listed in their spam folder. There are two ways that this can be changed so that it does not happen in future. This is through the use of the contact list and whitelist built into email clients.
Every email client is different but has the same functionality built into it.
An email contact list is a list of all email addresses that you send emails to. It also works in the opposite direction, such that any email address on your contact list will allow emails from them into your inbox. If a client was to add your email address to their contact list all your emails will go to their inbox and not spam.
A whitelist also known as an approved or safe sender list is a place where email addresses can be added confirming that emails from that source are shown in your inbox. If a client adds your email address to their whitelist your emails will never go to spam, instead straight to their inbox.