A few days ago, a number of people started reporting problems logging into FastMail. The symptom was the same, they could get to the FastMail login screen, but entering their username and password always returned a "The user name or password you entered was incorrect. Please try again." error. We did some checking, and all the accounts reported as affected appeared to be fine.
Well we finally tracked down the problem. It appears that in Firefox, if you just type "fastmail" into the address bar, since it's not a valid domain name, Firefox does a google search for "fastmail", takes the first result, and goes to that website. Normally that's been fine, because if you do a google search for "fastmail", http://www.fastmail.fm has been the first result. Additionally, if you try and login to the web interface using just the first part of your username (eg. joeblogs) rather than your whole username (eg. email@example.com), then we'll append the domain of the webpage you went to by default. So it seems a number of people with @fastmail.fm accounts, just type "fastmail" into the address bar, and then login using only the first part of their username, the bit before the @ which all worked fine.
Now for some reason a few days ago, google changed their search results. If you do a google search for "fastmail", they now return http://www.fmail.co.uk as the first result. That's one of our many domains, so what you see is still the same login screen. However, if you try and login with just the first part of your username, instead of assuming it's an @fastmail.fm domain account, it'll assume it's an @fmail.co.uk account. Of course in most cases the account won't actually exist, and you'll get the "The user name or password you entered was incorrect" error.
Now we know that google generally suppresses multiple pages that have the same content, which in the case of our sites is common because http://www.fastmail.fm, http://www.eml.cc, http://www.myfastmail.com, etc all point to our login page. We have no idea why google suddenly decided that http://www.fmail.co.uk was the preferred domain over http://www.fastmail.fm, and no idea how to convince them to change back either.
In the meantime, we'll make it so that when a login fails, it now shows:
"The user name (firstname.lastname@example.org) or password you entered was incorrect. Please try again."
Which should make it quickly obvious when there's a domain mismatch problem.
For people that are used to typing "fastmail" in the address bar to login, we recommend just typing "fastmail.fm" instead. It's only 3 extra characters, and will correctly take you to http://www.fastmail.fm where you can still use just the first part of your login name.