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Web Hosting and Cloud Computing Control Panels
Each migrated email account is wrongly "disabled". After selecting the account, in the "Email settings" -> "Primary email address enabled?", we can see that it comes selected to "no".
On an old migration I made a year ago, after changing it to "yes" solved the issue, but in the current case, even changing to "yes" isn't sufficient.
After the migration, first we need to change "Email settings" -> "Primary email address enabled?" to yes.
Only after we enable each account the login must work automatically. We only must change their passwords if, for some reason, the login doesn't work.
In my case, I changed the passwords of all accounts before realizing that I had to "re-enable" them.