Authentication is a way to prove an email isn't forged. SPF and DKIM are most common used authentication methods.
SPF (Sender Policy Framework) is a way for email service providers to verify that a mail server is authorized to send email for a domain.
DKIM is an authentication protocol that is used by email receivers to determine if an email message was sent by who it says it was sent by. DKIM often improves deliverability of emails to the inbox since a receiver can be confident that the message is not a forgery.
You will need access to your DNS server to make these updates or you will need access to a hosting panel that has access to edit your DNS records.
Below I share a video on how I do this and make sure to reference the text I have below to copy and paste into your TXT record on your DNS server.
Step 1: DKIM (Create a TXT record )
Record Type:TXT
HOST: em._domainkey
TXT VALUE: v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC75RQss0FsR1dIiQ8fJwD8RJlktEdllRbjHKZcMv8GBnCSqoyNzjuoA6Xn3sJkIw6ZR9eB4hiz1yEdJSVsQKnOCzfLfPFqFbC5z6fBNJ6h/JNHNkcRTaiSwQXxvtP4jpBRbzOoK8rxSDwvPITLhh63xgXomiGYmaMsWkIvVfBnhQIDAQAB
Step 2: SPF
If you don't have SPF records for your domain, then Create a TXT record
Record Type:TXT
HOST: @
TXT VALUE: v=spf1 include:spf1-srvs.em01.net ~all
Note: If you already have SPF record just add "include:spf1-srvs.em01.net" to existing SPF record.
Example: Lets say you have the folowing SPF record: v=spf1 +a +mx +ip4:122.133.144.155 include:spf.test.com ~all
After adding "include:spf1-srvs.em01.net" to existing SPF record you should have
v=spf1 +a +mx +ip4:122.133.144.155 include:spf.test.com include:spf1-srvs.em01.net ~all
NOTE: After you update your SPF you should wait 24 hours before sending due to DNS propagation.