User with two different X500 addresses | Trouble after installation Exchange 2016 | Recipient not found by Exchange Legacy

We installed complete new servers including new active directory and new Exchange server 2016. (Old was Exchange 2010). Emails were exported and imported.

After that sometimes internal mails couldn’t be delivert:

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'

The error occurred after answering to an old email.
We solved that problem by determining the legacy X500 addresses and adding them to the Exchange accounts proxyAddresses attribute. All worked fine afterwards.

Like here for "user1"

/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=<user name>

For "user1" now the error came again with a completely different X500 address:

/o=<company name> /ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=a6195749405b4b3daa678bdb0aee0358-<User1>

As far as I know Exchange can’t handle two X500 addresses.

Question
Is there any way to get generally rid of that problem?

Edit1
The X500 address for every user is already added to the proxyAddresses attribute.

My problem now is that there is at least one user who has two different X500 addresses. The first I found in the old Exchange LegacyExchangeDN attribute.
The second I found today in the “could not deliver mail”.

So I precise my question:

• Can exchange handle more than one X500 addresses?
• How can I find the other X500 addresses for users before an error
  occurs?