@Bhaski
Whether this user and other users in the same domain?
- Open IIS 6.0, check whether SMTP service is start. If it is started, please restart it.
- Go to Active Directory, check whether the user's email is filled in correctly.
- By default the Email property is mapping to the proxyAddresses attribute. Go to Active Directory, right click the user -> Properties -> Attribute Editor, check whether the user has value for proxyAddresses.
- Go to Site settings -> User alerts -> Choose the user and click update.
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