What does contain MailMessage.Sender property?
Maybe, the address you want is specified in the SENDER header.
If not, you have to parse the address from the mail.From property yourself.
In case of "Per conto di: bccroma@ciscrapec.it" this whole string is considered to be a Display Name for the SMTP address "posta-certificata@pec.aruba.it"
There is no build-in method for your task, but something like this should do the trick:
Dim address As String
If mail.From.Count = 0 OrElse
String.IsNullOrEmpty(mail.From(0).DisplayName) OrElse
Not mail.From(0).DisplayName.Contains(":"c) _
Then
address = String.Empty
Else
address = mail.From(0).DisplayName.Split(":"c)(1).Trim()
End If