Some time ago there was a post on reddit ( that 
described setting up an offline mail vm. Just kill the "send" part there 
and you'll get a mail black hole that receivs but never sends. Seems like 
this is more or less what you want.

On Tuesday, August 6, 2019 at 5:06:54 AM UTC+3, wrote:
> In Qubes, is it possible to set up a VM that can receive email, but not 
> send information out, via email or otherwise?
> The motivation is: Many online accounts rely on an email address to reset 
> passwords. However, the VM that handles inbound emails, processes a lot of 
> untrusted input. If the VM gets compromised by an attacker, the attacker 
> can then send password reset emails and read them. So to defend against 
> this, I want to prevent the compromised VM from communicating out the 
> contents of these password reset emails.
> Specifically:
> 1. Assume the VM is compromised (can't rely on in-VM enforcement 
> mechanisms).
> 2. Assume the email provider is not compromised
> To further illustrate the problem, here are example setups and why they 
> don't work:
> Setup 1: Use qubes firewall to restrict to the email provider's server and 
> IMAP port. Block UDP requests using qvm-firewall.
> Why it doesn't work: Attacker can create an account on the same email 
> provider and connect to their account (the firewall rules will not prevent 
> this). They can then sync emails containing any data, to their account.
> Setup 2: Like Setup 1, but use POP3.
> Why it doesn't work: Attacker creates account at provider, transmits data 
> via POP3 delete operations.
> Does anyone have a email setup with this inbound-only property, ideally 
> that does not require running their own email server?
> Thank you.
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