Hi Marco, 
On Sun, Feb 03, 2019 at 03:47 +0100, Marco Vittorini Orgeas wrote:
  [...] let's say user@domain1 is happily
exchanging
 mails with user@domain2, then user@domain1 starts to use delta to reply to
 user@domain2, over-taking ownership of future communications with
 user@domain2.
 
 What happens if user@domain1 want to continue using its MUAs to communicate
 with user@domain2 ? 
If user@domain1 mails with a non-DC MUA then those messages remain 
in the INBOX [*] -- so accessible to user@domain1's MUA.  They are also 
shown in DC as a chat -- is that what you take issue with?
Sharing DC and regular MUA usage with the same account are complex 
to handle because there are many different MUA setups out there. 
holger
[*] apart from problems arising from inter-MUA communication and DC bugs.