Due to the current situation with COVID-19 and many people now working remotely, some Microsoft services are throttling to handle the extra load.
This can result in a ‘503’ (Service Unavailable/Server Too Busy) is one of the errors that may appear when this occurs.

Due to this, the 503 error may also appear intermittently when using the OnePlaceMail App as well as OnePlaceMail & OnePlaceDocs Desktop with Office 365.

We have logged this with Microsoft Support and encourage users experiencing this to do the same via their tenant in Central Administration