I have two email accounts in the form email@example.com and firstname.lastname@example.org.
When I want send an email from the first account I get an authentification error, because the user name (=email address) is wrong. When I change it to the correct one (that with the "_") the username of the second account is also changed. So when I want to use the second account I get an authentification error again, because the username is wrong. If I correct that one (the one with the "."), the user name of the first one is also changed. And so I return to the the beginning of the problem " When I want to....."
Any idea what is going wrong?