Sender Policy Framework (SPF) allows receiving mail servers to authenticate whether the SMTP server originating the email is authorized to send email for the domain. Most email servers will check if the SPF record for a domain exists and if it doesn’t, there are chances that the email can get flagged as spam.
Until recently, I was using Mailjet as an SMTP relay server which allowed for upto 200 relays / day for free. I found out while trying to send email to Oster for a problem with our food processor, that my emails were getting bounced by their mailserver. And the worst part was, Mailjet did not notify me about it even though their dashboard indicated that the emails had bounced back. I decided that I would not use a mass mailing SMTP relay and would instead just use my service provider, Comcast for doing SMTP relays. Comcast does not allow for SMTP submissions on port 25, but use 587 like most other providers today.
I configured the SPF record to authorize my own mailserver to send out emails along with that of smtp.comcast.net. All other servers are disallowed from doing so via the -all directive:
"v=spf1 mx:mailserver.asrivas.me include:comcast.net -all"