This document describes how Service Desk populates the summary field on new tickets and how that works with the Email interface.
Service Desk requires the use of the %SUMMARY Text API keyword to populate the summary field on new tickets.
If that is not passed, then the first ~55 characters of the description field will be copied to the summary field when the ticket is created.
In most environments, the summary of tickets would be updated by a first line of support to reflect the current state of the ticket before it is transferred to the next level.
Also, it is possible to automatically attach the email to the ticket on ticket creation via the Mailbox configuration.
Another workaround would be to use the "Prepend Subject Line" feature on the appropriate mailbox rule.
See Figure 1, for example:
Example email as shown in Figure 2:
This will generate a ticker as shown in Figure 3: