Your problem is that you're relying on a method of communication that was never guaranteed to be instantaneous. Maybe you should investigate other forms of communication within your team besides (or in conjunction) with email.
Google processes a lot of email. The primary constraint limiting how long their MTA backlogs can be is customer satisfaction. Most SMTP servers I've worked on are happy to retry for 72 hours or more.
I try to treat email as any other letter I recieve(1). Meaning that the sender can only expect an instant reply if I know beforehand the mail is coming and urgent. The same way one would rapidly reply to an urgent letter one is expecting, but easily take a day or more to reply to, or even open, a non urgent letter.
If you need to get a hold of someone directly you should use your phone (to call them). If they don't pick up their phone, leave a voicemail or send an sms.
(1) Registration confirmations / new password emails are an obvious exception here.
[If at all; I know plenty of people who simply never check their voicemail...]