When Do I Use IP Address vs. Include vs. "a" Parameter In SPF Record For My Postfix SMTP Self Hosted Server?

You are correct that you can include either your mail server's FQDN (Fully Qualified Domain Name) or its IP address in your SPF record. Here are the two options you mentioned:

Using FQDN:

v=spf1 a:mail.mydomain.com ~all

Using IP:

v=spf1 ip4:11.11.11.11 ~all

Both of these choices are legitimate, and the decision between them is essentially a matter of preference, aligned with your specific server setup.

So what determines whether my client uses include_spf.mydomain.com, include:spf.mydomain.com, or include:mydomain.com? Whether my DNS (as the ESP company) has the ipv4 address or the "a" parameter listed in my SPF record?

The decision of whether to employ include:spf.mydomain.com or include:mydomain.com within your clients' SPF records depends on the way your SPF records have been structured. If you've created a distinct SPF record for your mail server, residing on a subdomain like spf.mydomain.com, then your clients should include this subdomain. Conversely, if your primary domain already encompasses the SPF record for your mail server (as is the case with mydomain.com), then they can directly include the primary domain.

But what happens when I now offer my email marketing as a SERVICE to clients for them to send emails from their domain on behalf of my smtp server. I need to give them an SPF record.

If you plan to offer this email marketing service to clients, each client should have their own unique SPF record that includes your SPF record for the mail server. Each client's SPF record should be tailored to their specific email sending needs, including any other authorized sources (e.g., their own mail servers if they have any).

For example, let's say your SPF record is configured like this for your central mail server:

v=spf1 a:mail.mydomain.com ~all

Client A wants to send emails from their domain (clientA.com) using your mail server. Their SPF record should include your SPF record as well as any other authorized sources, such as their own mail server if they have one:

v=spf1 include:mydomain.com include:clientA.com ~all

This SPF record indicates that emails from clientA.com are sent through both your mail server (mydomain.com) and their own authorized sources.

For the next client, you would replace "clientA.com" with "clientB.com" and so on.

This has been one the best response I have received to date! Very clear and understandable.

You explained it in dummy proof.

THANK YOU VERY MUCH!

Just a few follow up questions:

Regarding my SPF questions, when you say:
"The decision of whether to employ include:spf.mydomain.com or include:mydomain.com within your clients' SPF records depends on the way your SPF records have been structured. If you've created a distinct SPF record for your mail server, residing on a subdomain like spf.mydomain.com"…

What do you mean?

I cannot create an internal SPF record within my mail server's/postfix configuration files right?

All I know, whenever someone says "create an SPF record", my brain just registers this statement as the following:

1. Go to your DNS (e.g. mine is with Cloudflare).
2. Create a TXT record for your domain (news.mydomain.com)
3. Add either the a parameter or server's IP address for the value

In order to use the spf.mydomain.com, I would need to create a distinct SPF record for my mail server, residing on this subdomain. Thereby, my clients can then use it as in the case for Google.

But out of curiosity how would that be possible if the only two SPF variants that work for me (as confirmed by my email tests) are either using the a parameter (e.g. v=spf1 a:mail.mydomain.com ~all) or IP address (v=spf1 ip4:11.11.11.11 ~all)?

Last question.
I have other domains on Cloudflare. Havent tested this out because I am still finalizing the SASL authentication on Postfix.

If I am using this SPF record for my central mail server for this email sending domain: news.mydomain.com

v=spf1 a:mail.mydomain.com ~all

Are you telling me if I wanted to use this same mail server for another sending domain I own (news.mydomain2.com assuming I dont own a mail server for this subdomain), my spf record for this 2nd sending email domain would be:

v=spf1 include:mydomain.com ~all

In other words, this second sending domain's SPF record would be "pointing" to my mail server's domain (i.e.mydomain.com) since that's where I am sending emails from?

So all I need to do is add this SPF for news.mydomain2.com Cloudflare DNS and I am good to go? I wouldnt need to configure any postfix or mail server files to "add" the include:mydomain.com value for my clients/other domains?

Thank you again for your very detailed response!

Sorry but another question came to mind.

Let me highlight the domains:

mail.mydomain.com - my fully qualified domain name for my mail server
news.mydomain.com - one email sending domain (it shares the same parent domain as my mail server)

My DNS SPF setting for news.mydomain.com is
v=spf1 a:mail.mydomain.com ~all

If I was going to configure another subdomain to send emails from the same mail server (so now I have two email sending domains) which shares the same parent domain (e.g.info.mydomain.com), what would the SPF record look like?

Because I am sharing the same parent domain as my mail server (i.e. mydomain.com) would the SPF record look like this for info.mydomain.com?

v=spf1 a:mail.mydomain.com ~all

Or would it look something like this:
v=spf1 include:mydomain.com ~all

I think it's the former because the second sending domain shares the same parent domain as the mail server.

But I would love your confirmation on this as I am lost.

Thank you very much for your time and efforts!