Google Jail

Google hates spam and will put limits to accounts they believe are showing activities indicative of spamming. 

A sure indicator of this problem is when the account starts receiving emails from  nobody@gmail.com saying that the email could not be sent because the email limits have been reached. These errors do not indicate which prospects were not emailed and because of that, Quickmail cannot identify which emails bounced and will just continue sending follow-ups (which makes the errors more annoying).

It's almost all the time due to sudden spike in email volume or large email volume while the G Suite/Gmail account is still under trial limits.
It can also be caused by spam complaints, bounce rate or Google not liking the emails that are being sent out (spammy content).

Google will likely restore the account after 24 hours, but there needs to be a change in the email pattern to avoid the same issue.

Here are possible ways to fix the problem:

Lower the email volume
Around 20-30 total emails/day on the first couple of weeks is recommended.  Focus more on improving the email copy and value proposition to reduce the need to send out so many emails.  

You may not need to send emails by the hundreds everyday if your email copy is good and prospects are interested in learning more about your offering. You may then scale up slowly and gradually once you're already receiving responses and getting results. 

Don't use free @gmail.com email accounts
Free @gmail.com email accounts are easily flagged by Google and has very low sending limit. 
It's recommended to use a G Suite email account for better deliverability and higher sending limit.

Contact G Suite support and request to upgrade to 2,000 emails/day limit 
You may need to pay atleast $30 for that.  https://support.google.com/a/answer/166852?hl=en
If they say your email limit is already at 2000 emails/day - then you can ask them why they are limiting your email volume.

Check your audience/list and make sure they are well targeted and would actually like to get the emails you're sending out   
People tend to report emails they don't like as spam. 

Avoid copying templates from other users
Email templates used by spammers are flagged easily and Google doesn't like spam. 

!! Add more email variations to your sequence!! 
Add variations to your email steps and utilize attributes like {{prospect.first_name}} and {{company.name}}
to vary the subject lines and the email content. ({{firstname}} and {{company}} in the old version)

More email variations = better. You should make it appear that you're not sending the same emails to everyone in your list.

Increase the delay between emails
longer delay = better.
You can adjust the delay between emails by going to Inboxes -> Email Settings ->  then adjust the minimum delay between emails (Try 1-2 minutes) 

Refer to our checklist of better Cold Email practices
This nifty list can be found at  http://quickmail.io/checklist/ (a great source to bookmark!)

(Pro Only) Use Sendgrid to send your emails out instead of using Gmail's default mail sender.
Using Sendgrid, Google limits will no longer apply. 
You can check this guide for more info ->  http://help.quickmail.io/article/105-nv-custom-smtp-set-up

Note: Do not use gmail.com email accounts with SMTP as you'll surely get deliverability issues. You need to get a G suite email account under a custom domain if you want to use SMTP (like Sendgrid) to send emails out.