For our sins, we forward a certain amount of email to GMail (which is to say that it's sent to addresses here and then we send it onward to GMail). These days, GMail rejects a certain amount of that email at SMTP time with a message that some people will find very familiar:

550-5.7.26 This message does not have authentication information or fails to pass authentication checks (SPF or DKIM). [...]

(They helpfully include a link to their help section on "Make sure your messages are authenticated".)

As far as we can see from outside, there are two ways to pass this authentication requirement. First, the sending IP can be covered by actively positive SPF authorization, such as a '+a' clause. GMail actively ignores '~all', so I suspect that they also ignore '+all'. Second, you can DKIM sign your messages.

There are people who don't like email forwarding, but I can assure them that it definitely happens, possibly still a lot. Unless you want your email not to be accepted by GMail when forwarded, this means you need to DKIM sign it, because forwarded email won't pass SPF (and no, the world won't implement SRS).

GMail is not the only large email provider, but they are one of the influential ones. Where GMail goes today, others are likely to follow soon enough, if they haven't already. And even if other providers (or GMail) accept the message at SMTP time, they might use something similar to these requirements as part of deciding whether or not to file the new message away as spam.

I'm not really fond of the modern mail environment and how complex it's become. But it is what it is, so we get to live with it. If your mail system is capable of DKIM signing messages but you're not doing so yet, you should probably start. If your mailer can't DKIM sign messages, you probably need to look into fixing that in one way or another.

(We're lucky in that we're DKIM signing locally generated messages, and unlucky in that we do forward messages and so we're trying to figure out what we can do to help when the message isn't DKIM signed.)

Appending: The uncertainty of SRS and GMail

SPF's usual answer to how it breaks forwarding messages is SRS. However, it's not clear that SRS or any other scheme of rewriting just the envelope sender will pass GMail's SMTP authentication checks, because GMail's help specifically says (with emphasis mine):

For SPF and DKIM to authenticate a message, the message From: header must match the sending domain. Messages must pass either the SPF or the DKIM check to be authenticated.

SRS and similar schemes normally rewrite the envelope sender but not the message From:, and so would not pass what GMail says is their check (whether it actually is, who knows). Effectively GMail is insisting on DMARC alignment even without DMARC in the picture.