pigeonhole question: filtering on delivered-to in case of fetchmail

[@lbutlr]

> On 17 Jul 2019, at 10:03, Trever L. Adams via dovecot <dovecot at dovecot.org> wrote:
> 
>> On 15 Jul 2019, at 18:11, Trever L. Adams via dovecot <dovecot at dovecot.org
>> > wrote:
>> >
>>  So, one of the problems I am seeing is that people are trying to fake
>> 
>> >
>>  users into revealing information by sending from an outside domain but
>> 
>> >
>>  with an internal reply to address and claiming to be administration, IT
>> 
>> >
>>  or what not.
>> 
>> 
>> You should not accept external mail claiming to be from your domain unless that mail comes via authenticated submission. But if the reply to is going to an internal address… 
>> 
>> I’m puzzled by exactly what you mean here. Are you saying that users on your system are trying to phish other users on your system?
>> 
>> >
>>  I can set up something that will reject if from is outside the domain by
>> 
>> >
>>  reply to is internal. The problem is in some setups, there are fetchmail
>> 
>> >
>>  setups. I do not want to reject these with a message. Which is what I am
>> 
>> >
>>  currently doing for the others. Maybe I should discard them all without
>> 
>> >
>>  rejecting.
>> 
>> 
>> I haven’t used fetch mail in many many years, so I can’t answer anything specifically about it, but if you use it to allow external senders to send mail via your system in a way that is not authenticated then you should not do that.
>> 
> I do NOT allow email claiming to be from my domains. The problem is "forgery" of Reply-To headers. 

People are forging reply-to headers to go to local addresses on your system? What is the possible motivation to that? Anyone replying will not reach the spammer/phisher.

> Some nonsense about having failed to follow directions and if I don't click the link below, the account would be deleted. It was NOT talking about an account on another system, but the email account itself.

Ah, I see what the problem is now. This is a job for SpamAssassin. Or a milter to strip URLs )or render them uunlcikable) to external domain from animal with a reply-to-header in your domain. But what email clients show reply-to and not From? Heck, don't most mail clients not show reply-to at all?

> So, as you see, it is coming from an outside domain. As the sieve code showed, I am testing for where reply-to claims to be for internal domain, but the from is NOT from it. This email was a good example of that.

Yes, sieve would be ideal for this as it’s very easy to match that and then feed the message to your bayes filtering, but you have to make exceptions for mailing lists,a s they often have a from and a reply-to that are different.

I don’t think I’ve seen this behavior, and I still find it a bit weird. 


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