For the past several years, I have had my Apple-provided @me.com email address set up as an alias in Gmail, and enabled auto-forwarding of my iCloud emails to my Gmail account. This allows me to use Gmail as my one-stop-shop for sending and receiving emails from both my @gmail.com and @me.com addresses.
However, it recently came to my attention that many of my emails sent from my @me.com address via Gmail have automatically ended up in the spam boxes of my recipients—even those I've emailed regularly. This went on for a few weeks, with zero indication on my end, beyond a puzzling lack of replies.
Eventually, one of my recipients alerted me that my email went to spam, and I turned to Google to do some research. As it turns out, there is an industry-wide email authentication, policy, and reporting protocol named DMARC, and it appears Apple upped its DMARC policy to "quarantine" in July.
Essentially, this means that emails sent from an Apple-provided email address, such as @mac.com, @me.com, or @icloud.com, via a third-party email client such as Gmail, are now likely to be automatically marked as spam.
Al Iverson's Spam Resource explains:
If you monitor these things, you might have noticed that Apple's consumer email domains (iCloud domains) -- mac.com, me.com and icloud.com -- have moved to a "p=quarantine" DMARC policy. This means that if you have an email address in these domains, your ability to send outbound mail using an email service provider or other, non-Apple email platform to send mail, deliverability won't look so good. Mail may not be blocked outright (Apple didn't move to "p=reject") but moving to "p=quarantine" means it's much more likely that your mail could end up in the spam folder.
DMARC is designed to combat one of the most common types of phishing attacks, in which the "from" address in an email is faked, so Apple moving to a "quarantine" policy is a good move in terms of security, even if it is an inconvenience for people who use an Apple email via third-party clients.
After learning this, I reached out to Apple for clarification, and while it didn't confirm the new DMARC policy, it did offer a potential solution for Gmail.
Apple told me that I should be able to avoid the marked-as-spam issue by ensuring that emails from my @me.com address are set up to be sent through iCloud SMTP servers: smtp.mail.me.com. Apple has a related support document.
When I opened my Gmail settings, I discovered that my @me.com address was already configured in a similar manner, although the SMTP server domain was smtp.me.com, rather than smtp.mail.me.com. After updating it to the latter, emails from my @me.com address via Gmail began to reach the inboxes of others.
For further testing, I then reverted back to smtp.me.com, thinking that my emails would be marked as spam again. However, all of my emails still landed in the inboxes of others, including contacts I emailed for the first time.
At this point, I'm not entirely sure what has fixed the issue for me, but hopefully tinkering with the SMTP server settings works for others. If not, and you have an important email to send via your @mac.com, @me.com, or @icloud.com address, make sure to send it from Apple's own Mail app or iCloud.com.