Please also consider deliverability

Hi, cool article!

But speed is only one side of e-mail delivery. There are (lots of) methods to maximize your chances of having you e-mails delivered; fast or slowly but successfully delivered, which matters more.

Search the web with these keywords:
email deliverability spf can-spam reputation sender-id dkim (dk | "domainkeys")

Right, I understand that is part of the problem, and even put it in the text but those are not really PHP matters.

Well, I fear I've been misunderstood.
When one sends an e-mail, (s)he wants first the message to be actually delivered, and only secondarily that it arrives quickly.

So considering *only* the speed is not the major concern. Dealing only with PHP hides what is mainly necessary to have the message read... The sender would prefer it to arrive slowly in the inbox rather than finish quickly in the spam box of the e-mail service provider...

Respecting policies of e-mailing is not optional, it's a guaranty of long-term sustainability. See references in my previous comment.

Yes, I got your point. What I am trying to say is that matters that affect delivery speed can hardly affect deliverability.

That may the topic of a different article. Meanwhile you may want to take a look at the slides of this presentation that covers deliverability among other aspects.

phpclasses.org/browse/video/3/packa ...