At the time you got that error message, was Online.nl providing your
connection to the Internet? If not, you are running into a typical port 25
block. For more on that topic see
http://www.postcastserver.com/help/Port_25_Blocking.aspx
Were you sending while away from home, perhaps using a Wi-Fi connection?
The available workarounds when sending away from home are as follows:
1. Use webmail for sending (via your browser).
2. Ask your home ISP if they have a port other than '25' for SMTP.
3. If you use the same away-from-home connection frequently,
substitute the SMTP server belonging to that connection. This is a bit
tricky, but if you want to go that route, I can provide more details.
4. If you frequently send from multiple locations, get a free Gmail account,
configure it for POP access, set it up in Windows Mail, and use that for
your sending chores. Gmail uses port 465 for SMTP, which is not blocked
like port 25 is.