Google's SMTP server, smtp.gmail.com, has just expired a couple minutes ago. Here is the SSL certificate expiration notice:
It appears Google has forgotten to renew the certificate, even thought a Google Groups post had a warning to Google that it would be expiring today:
Is Google planning on updating the certificate for smtp.gmail.com port 465? I noticed that it's expiring today which could cause a lot of issues with email clients sending email today and tomorrow.
This is not the first time Google forgot to renew their certificates. They forgot to renew the AdWords conversion tracking certificate in the past as well.
Stuff like this happens. Google seems to be secure but they just forgot to renew the certificate.
Forum discussion at Google Groups.
Update: A Google spokesperson told us:
For a short time this morning, some Gmail users sending mail via POP and IMAP saw a notification on their mail clients that the SMTP certificate had expired. We identified the problem and fixed it promptly. We know how important Gmail is for our users, and we apologize for any inconvenience this may have caused.