The underlying connection was closed: This error message may occur when migrating a large item (containing large attachments, for instance). The Destination system will keep a connection open only for so long, after which the connection will be closed (timeout).
If the referenced server's configuration is under your control:
- Increase the connection timeout for incoming data transfers (see the sample screen shot below for IIS; use a value larger than 120).
- Increase the size limit of items that can be created through the API.
Once the mailbox migration completes:
To access the dialog below:
1. Go to the Exchange Server.
2. Start the IIS manager (run -> inetmgr.exe)
3. Browse to the EWS virtual directory.
4. Find the website under which it is located.
5. Right-click on Manage Website -> select Advanced Settings -> expand Connection Limits.