I received an email with the greeting
Guten [my name]
I know that Guten generally means good, but I’ve never seen it before a name; usually it is Guten Morgen for example. Can you please tell me what it means when used before a first name?
Using "Guten" before a name is always wrong, if "Gute(r)" ist not part of the name - and I don't know a name where it is*.
"Gute [female name]" or "Guter [male name]" is correct and could be used as a greeting, but it sounds strange, so hardly anybody will seriously do so.
A commonly used German greeting among friends is "Liebe(r) [name]".
*) Historically important people sometimes have bynames, for instance "der Alte Fritz".
There could be - theoretically - also someone "Good". I know "den Guten Heinrich", but this is some sort of spinach :-)
You can not use "Guten" like that. Most probably, the email you received was badly translated by some software.
Examples of how to usually adress people in letters or emails include (with increasing level of formality)
Hallo first name,
Liebe(r) first name
Liebe(r) Herr/Frau last name
Sehr geehrte(r) Herr/Frau last name
Of course it is also possible in emails to use
Guten Abend ...
Guten Morgen ...
Guten Tag ...
as eMails are considered sort of "real-time". You'd not use that in letters, as they take longer to deliver.
There is a very old way of adressing people using "Gute(r)", but that's so outdated nobody would ever use it today**.
Guter Mann, könntet ihr mir sagen ...
Guter Onkel, ...
Meine gute Mutter ...
** In prayers, "Guter Gott" might still be used today.
This is not realy a german greeting form for correspondence. The formal regards are "Mit besten Grüßen" or "Mit freundlichen Grüßen".
Not formal like "Mit besten Grüßen" is something like "Ihr [Name]" or "Deine [Name]". First form is for persons that you called by her family name.
In German: Das ist nicht wirklich eine deutsche Grußformel in der Korrespondenz. Die formalen Grußformeln sind "Mit besten Grüßen" oder "Mit freundlichen Grüßen".
Nicht formal, wie "Mit besten Grüßen" ist irgendetwas wie "Ihr [Name]" oder "Deine [Name]". Die erstere Form ist für Leute, die man nicht Siezt.