I am facing this same issue currently. This was working till some days back correctly. Now it is giving error whenever I execute the line:
var client = EWSClient.GetEWSClient(“https://outlook.office365.com/EWS/Exchange.asmx”, serviceId, servicePassword, domainId);
2019-08-05 07:36:46,668  ERROR - Getting error while retrieving mail details from GetEmails method.
System.Web.Services.Protocols.SoapHeaderException: SOAP header Action was not understood.
at System.Web.Services.Protocols.SoapHeaderHandling.SetHeaderMembers(SoapHeaderCollection headers, Object target, SoapHeaderMapping mappings, SoapHeaderDirection direction, Boolean client)
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object parameters)
at #=zQ6x2gZKDdkg2KO_xlBukqO4WBSnEMMYQuAyAc8IugWy0rJi0Uw==.GetFolder(GetFolderType GetFolder1)
at Aspose.Email.Clients.Exchange.WebService.EWSClient.GetEWSClient(String mailboxUri, ICredentials credentials, WebProxy proxy)
at Aspose.Email.Clients.Exchange.WebService.EWSClient.GetEWSClient(String mailboxUri, String username, String password, String domain)
I have observed the issue shared by you and request you to please first try using Aspose.Email for .NET 19.7 on your end. If there is still an issue then please provide the sample project along with test account access reproducing the issue so that we may investigate that further to help you out.