SMTP: Mail aliases and SPAM managing

[Magumo]

I make extensive use of email aliases to manage SPAM. Each alias is specific to an organisation and only provided to this one organisation. E.g. if I considered Admin a SPAM risk, I would set up admin@domain.co.za.

In the past the mail headers always included the alias to which a mail message was delivered. Thus I could see which address was receiving large volumes of SPAM and remove it.

Lately the mail header often does not show to which address the mail was delivered. Any reason for this?
The mail was delivered, so the address to which it was directed must be available.

Any way to determine to which of my aliases this was delivered?