Wrong reply address

[Marvin Oco]

In office 365, User smtp address is santos@contoso.com and receives mail in this address but when he replies, his reply address is sentos@contoso.com.

if we reply to sentos@contoso.com, we got NDR

any known issue?  tnx

[#2]

Mmmm...this is very strange: do you know if this user has additional aliases for his/her account?

[#3]

no aliases except for santos@contoso.onmicrosoft.com

user is syn ched via AAD