Hello, I use mailinabox for 12 months with a single domain.
In Mozila Thunderbird I use for the address of the server and smtp the address mail.mydomain.fr.
But today I have a new domain mydomain2.fr in Thunderbird I put mail.mydomain2.fr.
I create a mx parameter for the domain by redirecting mail.mydomain2.fr to mail.mydomain.fr but Thunderbird returns an identifier error, should I use as server address mail.mydomain.fr instead of mail.mydomain2. fr
Thank you in advance.
Your MX record and login details will always be the same whether you serve email for 1 or 20 domains.
So if your MX record is mail.mydomain.fr for your domain mydomain.fr it will be the same for mydomain2.fr or mydomain84.xyz.
What will change is the USER. The user will always be the specific complete email address.
I actually used the parameters of mydomain.com for mydomain2 and it works, but it seems to come from Thunderbird, because on my smarphone I use K9mail it uses automatically as address imap.mydomain2.fr and smtp.mydomain2.fr and it works also.
I specify that I created two recordings on my registar of type A. imap.mydomaine2.fr and smtp.mydomaine2.fr and redirected towards the ip of the mail server.
If the A record for the domain is still hosted on mail.mydomain.fr then you can just use mydomain2.fr as the MX and it will work.
Though, the proper configuration would be to use the A record of the MAIL SERVER not the VIRTUAL HOST. 
Just because something works, doesn’t mean that it is correct. As @murgero noted, there is a proper configuration.