User mailbox has more than one primary SMTP address

Today I've got this issue, an user who couldn't receive e-mails from anyone. When I checked his mailbox, EMC showed me an error saying that his account had more than one primary SMTP address, opening user's properties. EMC didn't let me remove any of them since you can't remove the primary.

Error message: "There are multiple primary SMTP addresses. Please ensure there is only one primary address for each address type."

So the solution I've found was to remove the extra SMTP addresses from the attribute proxyAddress on user's Active Directory object. Leave only one of the addresses with prefix SMTP.

UPDATE: Use ADSIEdit.msc tool to remove the extra SMTP addresses. More information about ADSIEdit here.

Then EMC will let you add the secondary SMTP addresses back and they won't be marked as primary.