Create custom mail header field

I'm making a program which sends invoices to registered users as PDF attachment. Each invoice is identified by a GUID. In some circumstance开发者_如何学JAVAs I want to be able to find programmatically the GUID from mail without reading PDF file.

Can I use mail header field to include invoice GUID, or it is a bad practice and mail header fields are not for this purpose?

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It is common practice to use "X-HeaderName" to designate non-standardized headers.