The same is happening to me also just browsing "sent items" folder trough IMAP in Thunderbird. The folder is not updated and I get the timeout error.
This is what is logged in SM:
[2018.11.28] 13:13:55 [x.x.x.x][18169847] Exception: (PooledTcpItem.cs) Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
[2018.11.28] 13:13:55 [x.x.x.x][18169847] StackTrace: at System.Net.Sockets.NetworkStream.BeginRead(Byte[] buffer, Int32 offset, Int32 size, AsyncCallback callback, Object state)
[2018.11.28] at System.Net.FixedSizeReader.StartReading() at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security._SslStream.BeginRead(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback asyncCallback, Object asyncState) at MailService.TcpServerLib.Common.PooledTcpItem.BeginReceive()