Actually this was the reason:
Dear FastMail.FM User,

You have just used our SMTP server. This service is not provided at
your membership level. Although you may use your email software to access
the service at all membership levels, you must upgrade to use SMTP.

Please see our help pages for complete details on how to setup your email
software to access your account to send and receive emails.

http://www.operamail.com/help/remote...and_ports.html

If you do wish to use this service, please upgrade your account to the
appropriate level. You can do this by logging in into the web site and
clicking 'Options' and then 'Upgrade' to upgrade your account, which
takes only a few moments.
Although if I do pay for the upgrade, will regular users running non elevated be able to send me email this way?