Wrongly addressed email receipts?

A short, practical explanation for @Basils57 (building on @Yzk's observation and @samaru's move): the address you see in Outlook Express is just a header the sender can set. Mail delivery is decided by the SMTP envelope (the RCPT TO address), not the visible "To:" field. That lets spammers send to many permutations or to catch-all mailboxes while showing a different recipient in the message text. See the SMTP spec for how envelope vs. headers work: RFC 5321.

Steps to check what actually happened:

• In Outlook Express open the message, choose File -> Properties and inspect the Internet headers box.
• Look for lines like Delivered-To:, Envelope-to: or X-Original-To: and the Received: chain. MTAs prepend Received: headers, so the topmost is the last hop and the bottommost is the origin server.
• If headers show the server accepted the message for your account, ask the mail host whether a catch-all or wildcard forwarding is enabled.

Mitigations that work in practice:

• Ask the ISP/host to disable catch-all addresses or to block known spam sources at the server.
• If you control the domain, publish SPF/DKIM/DMARC records to reduce spoofing (see DMARC RFC for details: RFC 7489).
• Use server-side spam filtering where possible. In the client, use cautious rules (but avoid deleting messages simply because your address is not in the visible To: — legitimate Bccs exist).

Caution: the From/To headers can be forged, so avoid replying to or opening attachments from suspicious mail. If troubleshooting, share full headers (with private lines redacted) with the mail host so they can trace the envelope recipient and delivery path.