Diagnose email that is not going out by analyzing if it is listed in blacklists or has wrongly set MX/SPF/DKIM records.
MX Records are DNS entries added to your domain that identify where to send your email to. This is typically the same provider as your inbox. MX records are not needed if your domain name is being used only to send out emails. It only counts for receiving.
SPF stands for Sender Policy Framework. In practice, it is a DNS entry added to your domain that lists servers and IP addresses that are allowed to send email using the domain.
When a server sends out an email, the receiving server will check if the SPF record lists the sender as a permitted sender. If it's not listed, the email can be marked as spam. This will largely depend on the receiving server settings.
In 2021, it is absolutely necessary to have a correct SPF record.
DKIM is a technique that verified that the email was not modified while in transit from the sender to receiver.
DKIM is similar to SPF - it is added as an additional DNS record for your domain. The receiving server reads this value and checks if your email passes.
In 2021 it is necessary to have correct entries for DKIM.