Spam sent from my email address
It is unlikely either your Mac or iPhone was used to send the e-mail. Regrettably it is difficult to protect yourself from this problem.

See Receiving spam from myself for a related question and answers about this subject. For a more technical overview, start with this spoof email search of the StackExchange network.

Spoofed E-mails

How Spammers Spoof Your Email Address (and How to Protect Yourself) – article from LifeHacks in 2014 helps explain the sitation:


  Most of us know spam when we see it, but seeing a strange email from a friend—or worse, from ourselves—in our inbox is pretty disconcerting. If you've seen an email that looks like it's from a friend, it doesn't mean they've been hacked. Spammers spoof those addresses all the time, and it's not hard to do. Here's how they do it, and how you can protect yourself.
  
  Spammers have been spoofing email addresses for a long time. Years ago, they used to get contact lists from malware-infected PCs. Today's data thieves choose their targets carefully, and phish them with messages that look like they came from friends, trustworthy sources, or even their own account.