Now, I have fixed the problem by using an other node.
Here is an example wich send an email when an OPC UA tag 'xSend' is set to TRUE, and it sets the OPC UA tag 'xSent' to true when the mail has been sent.
xSent returns to False when xSend returns to false.
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In the local server","justValue":true,"showStatusActivities":true,"showErrors":true,"parseStrings":false,"historyDays":"","x":442.0000190734863,"y":247.00000286102295,"wires":[["8b5a0cd8.fb222"]]},{"id":"8b5a0cd8.fb222","type":"OPCUA-IIoT-Response","z":"10110044.30b59","name":"Wait for answer","compressStructure":true,"showStatusActivities":true,"showErrors":true,"activateUnsetFilter":false,"activateFilters":false,"negateFilter":false,"filters":[],"x":672.0000152587891,"y":246.00000190734863,"wires":[["4abaa77f.5c3dc8","7c7aec60.dd4ea4","abdef645.a4af28"]]},{"id":"4abaa77f.5c3dc8","type":"switch","z":"10110044.30b59","name":"Test xSend = true","property":"payload[0].value","propertyType":"msg","rules":[{"t":"true"}],"checkall":"true","repair":false,"outputs":1,"x":881.0000190734863,"y":248.00000190734863,"wires":[["6198d7d9.ae24f8"]]},{"id":"7518e5a1.a4f6bc","type":"OPCUA-IIoT-Node","z":"10110044.30b59","injectType":"write","nodeId":"ns=5;s=Arp.Plc.Eclr/MainInstance.xSent","datatype":"Boolean","value":"true","name":"Send xSent ...","topic":"","showErrors":false,"x":1713.8868827819824,"y":302.45704460144043,"wires":[["e7f659ad.f856d8"]]},{"id":"e7f659ad.f856d8","type":"OPCUA-IIoT-Write","z":"10110044.30b59","connector":"21582029.3743a","name":"... to local server","justValue":true,"showStatusActivities":true,"showErrors":true,"x":1954.01997756958,"y":433.00394344329834,"wires":[["17416cc1.3e6113"]]},{"id":"17416cc1.3e6113","type":"OPCUA-IIoT-Response","z":"10110044.30b59","name":"Wait for answer","compressStructure":false,"showStatusActivities":false,"showErrors":false,"activateUnsetFilter":false,"activateFilters":false,"negateFilter":false,"filters":[],"x":2193.0200691223145,"y":433.00399684906006,"wires":[["33609b88.7c4cc4"]]},{"id":"33609b88.7c4cc4","type":"debug","z":"10110044.30b59","name":"","active":false,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":2380.0200958251953,"y":433.00395011901855,"wires":[]},{"id":"6198d7d9.ae24f8","type":"switch","z":"10110044.30b59","name":"Test xSent = false","property":"payload[2].value","propertyType":"msg","rules":[{"t":"false"}],"checkall":"true","repair":false,"outputs":1,"x":1117.019618988037,"y":248.00393104553223,"wires":[["33c9ab4f.6e6904"]]},{"id":"e4f13e7.aeee0c","type":"OPCUA-IIoT-Node","z":"10110044.30b59","injectType":"write","nodeId":"ns=5;s=Arp.Plc.Eclr/MainInstance.xSent","datatype":"Boolean","value":"","name":"Send xSent ...","topic":"ns=5;s=Arp.Plc.Eclr/MainInstance.xSent","showErrors":false,"x":1706.5197410583496,"y":431.00401973724365,"wires":[["e7f659ad.f856d8"]]},{"id":"bbf31571.bd4798","type":"change","z":"10110044.30b59","name":"set xSent = false","rules":[{"t":"set","p":"payload","pt":"msg","to":"false","tot":"bool"}],"action":"","property":"","from":"","to":"","reg":false,"x":1342.519832611084,"y":426.00395011901855,"wires":[["e4f13e7.aeee0c"]]},{"id":"7c7aec60.dd4ea4","type":"switch","z":"10110044.30b59","name":"Test xSend = false","property":"payload[0].value","propertyType":"msg","rules":[{"t":"false"}],"checkall":"true","repair":false,"outputs":1,"x":870.5195732116699,"y":423.0039119720459,"wires":[["bbf31571.bd4798"]]},{"id":"abdef645.a4af28","type":"debug","z":"10110044.30b59","name":"","active":false,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":1019.8945503234863,"y":638.6757850646973,"wires":[]},{"id":"4b371b6a.c3f7c4","type":"debug","z":"10110044.30b59","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":1576.01953125,"y":116.00390815734863,"wires":[]},{"id":"21582029.3743a","type":"OPCUA-IIoT-Connector","z":"","discoveryUrl":"","endpoint":"opc.tcp://127.0.0.1:4840","keepSessionAlive":true,"loginEnabled":false,"securityPolicy":"Basic128Rsa15","securityMode":"SIGNANDENCRYPT","name":"LOCAL SERVER","showErrors":true,"individualCerts":false,"publicCertificateFile":"","privateKeyFile":"","defaultSecureTokenLifetime":"","endpointMustExist":false,"autoSelectRightEndpoint":false,"strategyMaxRetry":"","strategyInitialDelay":"","strategyMaxDelay":"","strategyRandomisationFactor":"","requestedSessionTimeout":"","connectionStartDelay":"","reconnectDelay":"","maxBadSessionRequests":"10"}]