I ask because I have previously been given
email@example.com and I wondered if this is due to a sensible policy, or an unnecessary one.
Disallowing hyphens is rather unnecessary. Many email providers (including Hotmail) allow hyphens.
Broadening the scope of the question, is there any serious reason not to use any of the permitted symbols in an email address?
According to the standard,
"very.unusual.@.unusual.com"@example.com is a perfectly valid email address (souce).
But not all email clients adhere to the standard. Sending an email to this address is impossible for Windows Mail1.
Furthermore, many websites "validate" the provided email address in their forms. If your email address contains anything but alphanumeric characters, dots, hyphens and underscores, there's a good chance it won't pass some of those "validations".
1 Tested with version 6.0.6000.16386