You might have experienced the 550 5.4.5 error:
This appears in two instances:
When accounts hit their daily sending limit set by your email providers.
When this error occurs, we temporarily halt the sending account from sending any more emails to prevent any damage to your account or to ensure that it doesn't get suspended by Google.
This is also why you might not see that sending account send any emails recently, since it has been put on hold - this error can show up for even emails sent outside of Instantly from that specific sending account.
So, what we suggest you to do here is to reduce your sending volume to the recommended capacity (about 30) and then mark the account as fixed. This would resume sending emails.
If there's too much spammy content in your campaigns and leads have been marking your emails as spam.
When that happens, Google starts to soft lock your account and throttle the number of emails you can send with a third-party app (like Instantly) before you run into this error and hit your limits. You should also ensure this before marking your account as fixed.