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Old 16 May 2022, 11:52 PM   #1
paleolith
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redirecting to gmail?

I have an alias (in my own domain) which delivers to two end points, using Sieve. The first end point is a fileinto to a folder in my account. The other is a redirect to a gmail address of another person.

I have just discovered that some of the messages have not been delivered to the gmail address -- some are delivered, some are not. Also, that same partial delivery occurs if I "forward as attachment" to that address. The problem may be worse (I've only observed a small number) when the original sender is on gmail.

I assume that gmail is filtering it in some way due to being routed through Fastmail.

Any ideas on just what is happening, or what I might do about it?

Edward
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Old 17 May 2022, 03:25 AM   #2
hadaso
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Perhaps doing it not with Sieve, but with defining two targets for the alias would make a difference? (one target would be the gmail address; the other your Fastmail account, and then of course you can file it with Sieve).
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Old 17 May 2022, 06:57 AM   #3
xyzzy
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Have those gmail recipients checked their gmail's spam folder to see if this missing messages are in there?
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Old 17 May 2022, 12:55 PM   #4
n5bb
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Arrow Email forwarding can cause messages to become untrusted

I would guess that this is probably due to DMARC (DKIM and SPF techniques) settings at the domain of the original sender. See:

Fastmail help - Sender Authentication

Fastmail blog - SPF, DKIM & DMARC: email anti-spoofing technology history and future

As pointed out in those links above, some email senders (including some mailing lists) can break DKIM, and forwarding breaks SPF. DMARC requires address alignment, which usually fails when email is forwarded.

Unfortunately, current methods for automatic email forwarding and redirection can cause authentication (DMARC, SPF, DKIM, ARC, etc.) to fail at least partially. The sending domain DNS records can request that authentication be strictly applied. This can help with blocking spoofing senders of spam, but means that an email may be classified as spam (or discarded, depending on the final receiving server and the sending DNS authentication records) if the sender does any of these things:
  • Sending a message through a SMTP sending server with a From address whose DNS authentication records don't allow that server to be used. For example, sending a message with a From address at the gmail.com domain through a non-Google email system.
  • Automatically forwarding a message. If the message isn't properly DKIM signed, forwarding will (usually) break SPF and so cause DMARC to fail. Depending on the From sending domain DNS authentication records and the destination email server following those authentication rules, this can cause messages to be discarded, rejected, or filed as spam. If the forwarder changes any DKIM-signed content, forwarding can also cause DKIM to fail. If both SPF and DKIM fail, the message will nearly always be rejected or treated as spam by most modern email systems.
Gmail and Yahoo email were some of the first systems to become more strict about sender authentication due to the very high spam load those systems receive.

Bill
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Old 18 May 2022, 12:55 AM   #5
paleolith
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Thanks. I suspected that DMARC might be involved, and Bill explains the details.

After I figured out how to reach the spam folder in gmail, the recipient found the recent messages there.

I investigated gmail filters, and it looks like I'll be able to solve the problem with those. The emails in question always have the same To:, which gmail allows filtering on, and one of the optional actions is "never send to spam". Sounds perfect. Now I just need to explain how to do this to the non-techie recipient.

Edward
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