Since a couple weeks both clients and admins are not receiving e-mail anymore when an order is placed. Strangely enough I don't think any code or settings were changed around that time. I did a bit of testing to find out more about the issue:
-All possible payment methods don't send any e-mails anymore while they all used to do that.
- The following errors pop up after ordering something:


The controller for path '/Plugins/PaymentBuckarooIdeal/WebhookHandler/' was not found or does not implement IController.

System.Web.HttpException (0x80004005): The controller for path '/Plugins/PaymentBuckarooIdeal/WebhookHandler/' was not found or does not implement IController. at System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType) at System.Web.Mvc.DefaultControllerFactory.CreateController(RequestContext requestContext, String controllerName) at System.Web.Mvc.MvcHandler.ProcessRequestInit(HttpContextBase httpContext, IController& controller, IControllerFactory& factory) at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Page URL:  /plugins/paymentbuckarooideal/webhookhandler/


and

Error sending e-mail. Failure sending mail.

System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond [IP was here]:25 at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress) at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Exception& exception) --- End of inner exception stack trace --- at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6) at System.Net.PooledStream.Activate(Object owningObject, Boolean async, GeneralAsyncDelegate asyncCallback) at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate asyncCallback) at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout) at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message) --- End of inner exception stack trace --- at System.Net.Mail.SmtpClient.Send(MailMessage message) at Nop.Services.Messages.EmailSender.SendEmail(EmailAccount emailAccount, String subject, String body, String fromAddress, String fromName, String toAddress, String toName, String replyTo, String replyToName, IEnumerable`1 bcc, IEnumerable`1 cc, String attachmentFilePath, String attachmentFileName, Int32 attachedDownloadId) in \Libraries\Nop.Services\Messages\EmailSender.cs:line 130 at Nop.Services.Messages.QueuedMessagesSendTask.Execute() in \Libraries\Nop.Services\Messages\QueuedMessagesSendTask.cs:line 43


- An offline version sends emails correctly. This version however has different plugins. This is why I assumed the problem lies within a plugin. However, since all payment methods are affected, which rely on different plugins, this doesn't seem to be very likely either.

I'm quite stuck on this, if any more information is required just let me know!