FluentMail.Mailgun: System.Net.Sockets.SocketException: Resource temporarily unavailable

[thomasd3]

I have the following crash:

fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[1]

      An unhandled exception has occurred while executing the request.

System.Net.Http.HttpRequestException: Resource temporarily unavailable ---> System.Net.Sockets.SocketException: Resource temporarily unavailable

   at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)

   --- End of inner exception stack trace ---

   at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Net.Http.HttpConnectionPool.CreateConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Net.Http.HttpConnectionPool.WaitForCreatedConnectionAsync(ValueTask`1 creationTask)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
   at FluentEmail.Mailgun.HttpHelpers.HttpClientExtensions.PostMultipart[T](HttpClient client, String url, List`1 parameters, List`1 files)
   at FluentEmail.Mailgun.MailgunSender.SendAsync(IFluentEmail email, Nullable`1 token)
   at FluentEmail.Mailgun.MailgunSender.Send(IFluentEmail email, Nullable`1 token)

It is with .NET Core 2.1.6 and FluentMail.Mailgun 2.5.3.

On my Mac it works, but on CentOs I get the crash; what could be the issue?