http status code 503 - Httpclient rest graph api get error The application request queue is full - Stack Overflow

When making a REST API call to .0/me/mailFolders/Inbox/messages?$top=10&$orderby=receivedDateTime , I get the error message:

Application Request Queue Full HTTP Error 503. The application request queue is full.

What is causing this error and how can I prevent it? This is the code where I make the call. The error occurs on the last command.

var apiUrl = $".0/me/mailFolders/Inbox/messages?$top={RecordPerTime}&$orderby=receivedDateTime asc";

var mailResponse = await httpClient.GetAsync(apiUrlz);
var mailResult = await mailResponse.Content.ReadAsStringAsync();