To help ensure emails sent out from Moodle / Totara do not get marked as spam is to not use fake email accounts with live domain names (URLs) in the profiles of user accounts on Moodle / Totara.
The reason for this is when the Lambda mail server sends out emails to accounts that don't exist it ends up looking like we are sending out spam to a random list of emails in an attempt to find a real email account.
A step that can be taken to help Lambda not end up on the email Blacklist is to not use any fake email accounts with place holder email addresses for users or to update existing fake email addresses to a real email account.
An alternative for situations where the user does not have a real email account would be to use a fake email account for a domain name that does not exist like example.com.
For example email@example.com, firstname.lastname@example.org
Another suggestion is to setup an SPF record, which more can be found on here.
Please ask the Lambda Support team if you have any questions or want help figuring out if a domain name is live or not.