Exchange 2013 EWS large files send error

Hi there,

I have a strange problem with EWS sync in EM Client. It works reliable, sync folders, download and send messages, but i can’t send large messages (approximately 10Kb and more). When I try to send such message, this error logged:

12:26:50 PM postmaster@wolf.ru [Exchange Web Services]  Sending email12:26:50 PM postmaster@wolf.ru [Exchange Web Services]  Sending email: test 1
12:26:51 PM postmaster@wolf.ru [Exchange Web Services]  MailClient.Accounts.ConnectionException: The underlying connection was closed: An unexpected error occurred on a send. —> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. —> System.IO.IOException: Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host. —> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
12:26:51 PM    at System.Net.Sockets.Socket.MultipleSend(BufferOffsetSize[] buffers, SocketFlags socketFlags)
12:26:51 PM    at System.Net.Sockets.NetworkStream.MultipleWrite(BufferOffsetSize[] buffers)
12:26:51 PM    — End of inner exception stack trace —
12:26:51 PM    at System.Net.Sockets.NetworkStream.MultipleWrite(BufferOffsetSize[] buffers)
12:26:51 PM    at System.Net.Security._SslStream.StartWriting(SplitWritesState splitWrite, SplitWriteAsyncProtocolRequest asyncRequest)
12:26:51 PM    at System.Net.Security._SslStream.ProcessWrite(BufferOffsetSize[] buffers, SplitWriteAsyncProtocolRequest asyncRequest)
12:26:51 PM    at System.Net.TlsStream.MultipleWrite(BufferOffsetSize[] buffers)
12:26:51 PM    at System.Net.Connection.Write(ScatterGatherBuffers writeBuffer)
12:26:51 PM    at System.Net.ConnectStream.ResubmitWrite(ConnectStream oldStream, Boolean suppressWrite)
12:26:51 PM    — End of inner exception stack trace —
12:26:51 PM    at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
12:26:51 PM    — End of inner exception stack trace —
12:26:51 PM    at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
12:26:51 PM    at MailClient.Commands.Command.Process(WorkerStatus status)
 
We have somewhat complicated Exchange cloud setup (with SQUID reverse proxies and DAG, so it is not single server).

Initially we had this same problem with Office 2016 for Mac.  But when login set in login@domain.fqdn format instead of domain\login, it start working (sending large messages) even without restarting Outlook. 

It MAY be setup problem, but it is not appearing in test apps or log errors. 

Sadly, in em client it does not work either with login@domain.fqdn or domain\login format. 

As a EWS experts, you may know what can possible be the issue here? Do you authenticate according to typed data or parse the username? What can cause this?