SMTP host name vs. domain in "From:" address vis-a-vis Email Deliverability

I'm trying to implement (or make sure that I'm correctly following) email sending best practices to improve deliverability, but the role of the smtp server's host name vs the domain name of the From: email address seems to be unclear, even after reading dozens of people's articles/input.

Specifically, I understand that to satisfy the reverse DNS check, there must be a PTR record for the IP address of the sending machine that yields a domain name that matches the host name of the sending machine / SMTP server. Some say it needs to match the one given by the "hostname" command, most say it's the one provided with the HELO / EHLO statement, and this guy even says they MUST be the same (according to / enforced by what, I don't know; that's only a minor point of confusion, anyhow).

First, what I can't find anywhere is whether or not the domain name of the From: email address needs to match the domain name of the SMTP server.

So in my case, I have a VPS with linode. It primarily hosts a particular domain of mine, example.com, but I also sometimes do work on other projects: foo.com and bar.com.

So what I'm wondering is if I can just leave the default linode PTR record (which resolves to abc.def.linode.com), make sure that abc.def.linode.com is what my mail server (qmail) is configured to say at HELO, and then proceed to use it to send out emails for example.com, foo.com, et al.

If so, then I am confused by the advice given here, specifically (in a listing of bad case scenarios):

No SPF record for the domain being used in the HELO command

Why would THAT domain need an SPF record? And if it does, which domain should it provide whitelisting for: the HELO domain, or the domain of the From: email address (envelope sender)? Also, which domain would need to accept mail sent to postmaster@domain.com?

If the domains must be the same, that would seem rather limiting to me, because then for every domain you wanted to send email from, you'd have to get another IP address for it. It would also compromise or ruin one's ability to do non-email sending things (e.g. wget) relatively anonymously. However, the upside--if this is the case--is that it would make for a far less confusing setup.

I'm currently using the linode.com SMTP+PTR domain and example.com From: address combination without much of any deliverability issue, but my volume is very low and I'd like to know if someone out there has experience with larger volumes and has specifically tested the difference and/or has inside knowledge and/or has an authoritative answer (and source) for this particular question. I'm happy to clarify anything, let me know. Thanks in advance.