I am also have this exact issue - the emails appear to be going out but every email I send defaults to this page HTTP Status 400 – Bad Request Type Exception Report Message Request header is too large Description The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing). Exception java.lang.IllegalArgumentException: Request header is too large org.apache.coyote.http11.Http11InputBuffer.fill(Http11InputBuffer.java:770) org.apache.coyote.http11.Http11InputBuffer.parseHeader(Http11InputBuffer.java:964) org.apache.coyote.http11.Http11InputBuffer.parseHeaders(Http11InputBuffer.java:591) org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:286) org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:928) org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1794) org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Thread.java:750) Note The full stack trace of the root cause is available in the server logs. Apache Tomcat (TomEE)/9.0.82 (8.0.16) ... View more