I recently received this email from Gmail regarding an account I use to send emails from a client's site.
I panicked, I didn't panic. Google was clearly blocking that sign-in attempt so there wasn't much to worry about. All I needed to do was log in and change the password. Coincidentally, but not really, the client emailed me at the same time saying the contact form on his site wasn't working! Seeing as how his company is very client-centric, this was a significant problem.
The site is built using django so I dropped into the django shell and sent an email from the command line. This was the error I received:
SMTPAuthenticationError: (534, '5.7.14 <https://accounts.google.com/ContinueSignIn?sarp=1&scc=1&plt=AKgnsbsVh\n5.7.14 bOekrcadOp6yW7TnFXmLuByKYG95tri4I_HrNxbbkdvnuSE5ni6o331QnAUXg59sCywCxq\n5.7.14 yBe99ymMSUTOSDeW1M_oypqtkGMU0CD10ZNclSiOpgdtsL1kvqcbgCqoH90o8hvGDYx0jM\n5.7.14 PNELrv1CTfsy9Ufd23rB_e6ZB5NoEWUfgTKGmBhcSoOX7TmkettGMBBChmStac3gSzYyjt\n5.7.14 f2OK_zw> Please log in via your web browser and then try again.\n5.7.14 Learn more at https://support.google.com/mail/bin/answer.py?answer=787\n5.7.14 54 ka1sm487617igb.7 - gsmtp')
It seems that Gmail adds an extra level of security to your account when it prevents a suspicious sign in attempt. One side effect of that security is that the smtp functionality gets blocked/disabled.
I simply had to change the account password and all was well.
One a side not, I am quite impressed by google's dedication to the security of user's accounts. Account activity monitoring and automatic security precautions for a free, almost zero-traffic, account? I'll take 10+ accounts please, thanks!