# Exceptional Cases

This section describes some exceptional cases that you must be aware of when you use Substation ES.

## IMS System Response and Non-reply Messages

When you are using the IMS transaction method of invocation, and do not receive an expected reply message, the TIBCO messaging application might expect to receive IMS system responses.

To allow for this situation, the recipe must define the outbound conversion rule with a large enough buffer size to accommodate the IMS system responses. Alternately, you can define an extra field and map it as an optional field at the end of the inbound buffer.

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