This is an error that originates from Salesforce and is caused by exceeding your Salesforce-allotted API limit. The API limits refresh once daily.
Whenever Yesware makes a 'call' or communicates with your Salesforce organization, we will use API calls. This would include actions such as using the Salesforce sidebar to look up Accounts/Leads or syncing Yesware tracked emails into Salesforce. Depending on the type of action, there might be several API calls made. If you only want certain events in your Salesforce org, your account manager can request to not have open events, clicked link events, etc. synced into Salesforce. In addition, your Salesforce Admin can check the API call usage in Salesforce and if needed, request a limit increase with Salesforce.
If you'd like to increase the API limit, reach out to Salesforce directly. Please note that increasing the API limit will likely carry an extra monthly charge.