Imac Mail And Google Server Not Connecting

Can't send gmail from iMac Mail.  Receiving fine.  Sending using Safari is fine.  But I like iMac Mail because I can toggle between all three of my gmail addresses without having to log onto each one as I must if I use Safari.  Suddenly get error message when trying to send using Mail.  Error message. Server and Mail application not connecting.  Tried copying and rebooting Mail.  Works ok few messages, then same problem.  Help???