Dim Email As Outlook.MailItem
Dim domaintype As String                ' domaintype is given
'   domaintype = "@aaa.com"
'   domaintype = "@bbb.com"
Dim recipientname As String             ' recipient name is partially known
'   recipientname = "John Doe"

if domaintype = "@aaa.com" then
    With Email
        ' How do I add the correct recipient?
        .Recipient.Add recipientname    ' SMTP address = john@aaa.com for "John Doe"
    End With
elseif domaintype = "@bbb.com" then
    With Email
        ' How do I add the correct recipient?
        .Recipient.Add recipientname    ' SMTP address = doe@bbb.com for "John Doe2"
    End With
else
    ' do something else
end if

Email.Recipients.ResolveAll     ' "John Doe" is not resolved because it is ambiguous!