It might have something to do with your spamfilter.
My filter sent me this warning:
Spam detection software, running on the system "your.server.com", has
identified this incoming email as possible spam. The original message
has been attached to this so you can view it (if it isn't spam) or label
similar future email. If you have any questions, see
the administrator of that system for details.
Content preview: Hi test2, Here is your password reset
link[http://localhost/xxxx/Security/changepassword?h=xxxanidentifierxxx]
for http://localhost/xxxx/. Hi test2, [...]
Content analysis details: (6.5 points, 5.0 required)
pts rule name description
---- ---------------------- --------------------------------------------------
0.0 MISSING_MID Missing Message-Id: header
2.2 FH_FROMEML_NOTLD E-mail address doesn't have TLD (.com, etc.)
0.0 HTML_MESSAGE BODY: HTML included in message
1.8 MIME_QP_LONG_LINE RAW: Quoted-printable line longer than 76 chars
2.5 DNS_FROM_RFC_DSN RBL: Envelope sender in dsn.rfc-ignorant.org
The original message was not completely plain text, and may be unsafe to
open with some email clients; in particular, it may contain a virus,
or confirm that your address can receive spam. If you wish to view
it, it may be safer to save it to a file and open it with an editor.
Thankfully I could still open it. But it indeed is kind of annoying and might be a bug.
You'd better file it as a bugreport at open.silverstripe.org.