Migration users from Active Directory to Exchange Online fails

[AtanasM]

I am trying to migrate users from Active Directory to Exchange Online, but the following error message appears:
Error: MigrationPermanentException: The target mailbox doesn't have an SMTP proxy matching 'domain.mail.onmicrosoft.com'.
Tried to add the alias "email@domain.mail.onmicrosoft.com on the Active Directory as proxy address for the user, but it was not possible.
Error message:
The address 'smtp: email@domain.mail.onmicrosoft.com' is invalid. The specified argument is outside the valid range of values. Parameter name: The address 'email@domain.mail.onmicrosoft.com' is not a valid SMTP address.
What is the reason for this? How can I solve this issue?

[#2]

Generally speaking, you'd want to adjust your email address policy to include the domain.mail.onmicrosoft.com addresses. https://docs.microsoft.com/en-us/exchange/troubleshoot/move-or-migrate-mailboxes/no-smtp-proxy-match...