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I want to send a bunch of E-Mails to different recipients. My sending server (myserver.com) ist set up correctly, i.e.:

1. EHLO command, hostname and A DNS record do match
2. PTR record matches hostname and A record

The E-Mails I send are for a 3rd party. Thus, the "mail form:" should include a domain which I don't have access to (xyz@other.com). With my understanding, this should not be a problem as long as the "envelope from:" sender corresponds to my sending server, i.e. somebody@myserver.com Also, I would have to account for any SPF entries in the DNS records of other.com
Since, other.com does not specify and SPF entry this should be fine as well.

Now: Is there anything wrong with this approach? Are there receiving servers which would not deliver my E-Mail? Is there an increased probability that my E-Mails will be marked as spam?

I ask this, because so far from a technical standpoint I don't see any problems with this procedure. On the other hand, it seems that this would make it far too easy to "fake" E-Mails. So this gives me some doubt. Still, this shows why it is important to use an SPF entry, I guess.

  • That’s basically what Amazon SES and marketing companies like ConstantContact do. Spam prevention techniques are an art form that most providers don’t share the details of. Whether or not you will have delivery problems or not is completely a guess at this point. – Appleoddity Dec 22 '17 at 15:21
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Now: Is there anything wrong with this approach? Are there receiving servers which would not deliver my E-Mail? Is there an increased probability that my E-Mails will be marked as spam?

You have guessed it all. There is problem with this approach, basically there are no means for receiving party to distinguish whether you are legitimate sender or reckless domain-faking spammer.

Even when you do not use niether SPF or DKIM/DMARC recieving party may at least know that email is coming from domain's A or MX record designated host. As this is, as far as I understand your question, not your case, your email massages would be treated bad from the start.

Always try to protect your MTA reputation by sending only from domains that it may be associated with by DNS settings (A, MX, SPF, DKIM). Otherwise, your reputation shall not last for long.

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