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Old 9 Dec 2022, 09:57 PM   #1
evfrson
Senior Member
 
Join Date: Oct 2015
Location: London/Tokyo/Dubai
Posts: 144
Disable subdomain addressing for an alias

I seem to remember that there is an option somewhere to disable the receiving of email for a particular alias that is using subdomain addressing for that alias.
Problem is I cannot find any such option. I have looked in the obvious places and at Fastmail help but cannot find it.
Is/was there such an option or am I imagining things ?
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Old 9 Dec 2022, 11:16 PM   #2
Avion
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For an existing alias, go to Settings/Users & Aliases.

Find the alias, then Edit/Show advanced preferences.

Enable check/tick the 'Reject' option.
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Old 10 Dec 2022, 04:08 AM   #3
n5bb
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That is indeed how you can disable a particular alias, including all associated subdomain aliases.
  • For example, let’s say you have control over the alias “store@example.com”. If you disable this alias as described in the previous post, then email sent to store@example.com or any associated subdomain address such as good@store.example.com or bad@store.example.com will not be accepted by the Fastmail incoming SMTP server. The sending system will treat that as a non-existing email address and the sending system will inform the sender directly of the error. No emails will be transferred.
  • But let’s say you only want to disable certain subdomain aliases (bad1 & bad2 @store.example.com) and still receive all other email associated with the store@example.com alias. You can’t disable the store@example.com alias or you will block all associated subdomain addresses.
  • To ignore messages sent to certain subdomain addresses, you can simply create a normal rule which will treat only the subdomain addresses bad1@store.example.com and bad2@store.example.com as spam or just delete such messages.
  • In all of the examples above, no rejection message is sent to the sender. This is the best solution in most cases, since spammers sending Joe Job Spam can’t bother an innocent third party due you your rejection of the address.
In a few cases (such as when you are reasonably sure that a human will read any responses to their message to you) you might want to automatically reply to the sender with some type of rejection announcement. In this case actual emails are sent both to and from your account, which means that your rejection message could cause spam to others if a Joe Job attack is used by a spammer, and it’s even possible that your rejection message could be marked as spam and lead to future difficulties for you. If you understand these issues you can use the following custom Sieve code in your Rules. You should edit the example code below with the desired email addresses and insert it near the start of your code somewhere before step “###4. User Configured Rules”. You can change the rejection message to add a new email address you want that sender to use or other information.

Code:
if header :contains  ["to", "cc", "resent-to", "x-delivered-to"] [
“bad1@store.example.com",“bad2@store.example.com"] {
  reject "Mailbox does not exist";
  stop;
}
Bill
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Old 10 Dec 2022, 04:51 AM   #4
evfrson
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Join Date: Oct 2015
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Thanks both for your replies and in particular Bill for your always excellent replies.
I will just create simple rules to delete the subdomain messages.
It would have been better if there was just a global option to reject all subdomain addresses but there isn't so I will have to live with it.
The spammers are using different subdomain addresses each time (different values for bad in your reply above) so a specific rule may be pointless because the address in that rule probably won't be used again.

Last edited by evfrson : 10 Dec 2022 at 05:18 AM.
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Old 10 Dec 2022, 12:06 PM   #5
BritTim
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Quote:
Originally Posted by evfrson View Post
It would have been better if there was just a global option to reject all subdomain addresses but there isn't so I will have to live with it.
The spammers are using different subdomain addresses each time (different values for bad in your reply above) so a specific rule may be pointless because the address in that rule probably won't be used again.
Be aware that it is possible to create a rule that will reject all subdomains of a specific alias, .while still accepting messages sent to the alias without using a subdomain. The rule does not have to target just a single email address. It can identify email addresses matching a pattern. It can even be a regular expression.
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Old 10 Dec 2022, 07:48 PM   #6
evfrson
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Join Date: Oct 2015
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Quote:
Originally Posted by BritTim View Post
Be aware that it is possible to create a rule that will reject all subdomains of a specific alias, .while still accepting messages sent to the alias without using a subdomain. The rule does not have to target just a single email address. It can identify email addresses matching a pattern. It can even be a regular expression.
That's just what I need. I will look at the Fastmail help and play around with the rules. Thanks for your help.
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Old 11 Dec 2022, 11:30 PM   #7
hadaso
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I think that if the alias is in a user's own domain, one can set a bogus MX record for all subdomains, or for particular subdomains (i.e., refer the sender to a server that will reject the incoming email, or that does not exist at all. This would reject an email and would create a failure message for legitimate senders that would tell them the delivery failed, but would not create back-scatter as a result from a Joe Job attack as the delivery failure message is created by the sender and not by the receiver.
In case of a particular alias under a user's own domain, one can also create an NS record for the particular subdomain, then import the subdomain as a domain into fastmail, and then create aliases under that subdomain that can be set to anything that an alias can do, including disabled status, which would then reject all mail to that alias. For example, a user that has the privete domain example.org and wants to reject all email to addresses bad1@store.example.org and bad2@store.example.org would create an NS record for store.example.org pointing to Fastmail's name servers. Then import the domain store.example.org to Fastmail. Then create the aliases bad1 and bad2 in this domain and disable them (so they rejsect all email sent to bad1@store.example.org and bad2@store.example.or, and also to all addresses in the subdomains bad1.store.example.org and bad2.store.example.or). One would also probably set a catchall alias for the domain store.example.org so all email sent to other email addresses in this domain and it's subdomains will be received.
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