Cite it exactly like you would a source for which you have a full name, except the name is "Anonymous."
"XXXXXXXXXXXXXXXXXX." (Anonymous, 2019)
If it comes from a published work that has no author, you can use the name of the work in lieu of the author's name.
Both APA and MLA use both of the above methods.
Instead of "Anonymous" you can use "Unknown" or "Unknown Author" or "Source unknown." Etc.
If you don't know the exact date, don't make one up. But you might know the general time period. Or the location and other details. For example:
- Unknown author, 17th century Kenya.
- Traditional Italian folk tale.
- Graffiti, London, 1960's.
- Anonymous saying.
The poster has since clarified:
I'm quoting from a document that was sent to the person that I'm
writing the email to. I need to clarify something, and to do so I need
to quote from a section in the document. The document and the section
have names, but the creator of the document is unknown.
In this specific case, I would suggest one of the following:
- (Section) in (document) states: "XXXXXXXXXXXXXXXXXX."
- As described in the (section) of (document), "XXXXXXXXXXXXXXXXXX."
- "XXXXXXXXXXXXXXXXXX." ((document))