As of a few days ago, Safari 9.1.3 on OSX 10.9.5 has been unable to login to Gmail in standard mode. This is likely something which may not be fixed by Google, as they only promise to provide support for current and 1 previous version of Safari (At this time that would be 11 and 10), so not likely to see this bug fixed. If you are still running 10.9.5 you will need to upgrade to a new OSX which supports a newer version of Safari or use an alternative mail client or web based email in the current Chrome or Firefox browser.
Hope this helps someone understand what is happening, and save some time troubleshooting. Brad -- You received this message because you are subscribed to the Google Groups "mailplaneapp" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/mailplaneapp. For more options, visit https://groups.google.com/d/optout.
