Email addresses associated with an user are the values of mail, otherMailbox and proxyAddresses attributes. The last two attributes expect a smtp: prefix before the actual value. Also userPrincipalName is considered an email address, by default; this can be disabled (see commands below).
I see that userPrincipalName is used but when I write a new mail, recipient addresses are searched only in mail field… is it (the field) mandatory?