The old adage "There are only two hard things in Computer Science: cache invalidation and naming things." should really be revisited, because it's obvious that there are three hard things in Computer Science: cache invalidation, naming things and e-mail format validation.

There have been so many attempts over the years to write a concise set of rules and regulations for what is and isn't a valid e-mail, but the only real authorities are things like RFC 822 (section 6.1) and so on.

My own implementation of a "valid" email address resolver looks like this:

• A "local" part (before the ´@´) that contains any ASCII character, including control characters.
• Certain ASCII characters (spaces and the following: "(),:;<>@[\]) in the "local" part can only be allowed inside quoted strings.
• The "local" part may not begin or end with a period (.)
• The "local" part may not have two periods in a row at any point, unless inside a quoted string.
• The "domain" part, in full, has to resolve to an MX record in a DNS lookup or, if an MX record is missing, an A record for said domain.

And in this case, while your address passes the first four rules, it fails the fifth and final:

❯ nslookup
> set type=mx
> forum.dk
Server:		10.0.1.2
Address:	10.0.1.2#53

Non-authoritative answer:
forum.dk	mail exchanger = 10 forummail.forum.dk.

> forum.dk:frame13531
Server:		10.0.1.2
Address:	10.0.1.2#53

** server can't find forum.dk:frame13531: NXDOMAIN