Standardly, the Subject for emailed Sales Order using "To Be Emailed" is defaulting to "Company Name: YYYY SO#". By achieving the change of the subject, customer can provide more detail into the subject and may easily send the emails without them manually setting the field mentioned using Email button (Transaction > Communication subtab > Email button).
User can rename the "Company Name" field, which changes only the first part of the Email Subject. More information regarding this can be found under SA Article ID: 30117 Ability to change the subject of Sales Order confirmation email.
If you would like to replace "YYYY" part of Sales Order Email Subject, user can perform following steps and rename the "From Title" field on your preferred Transaction Form.
Please perform following steps:
1. Navigate to Customization > Forms > Transaction Forms.
2. Edit the preferred Sales Order form.
3. PRINTING TYPE > select BASIC.
4. Printing Fields subtab > Header.
5. Form Title field, LABEL = Enter desired label (this will replace the "YYYY" part)
Note: Mark the PRINT/EMAIL checkbox for this.
The number of Sales Order "SO #XXXX" will stay in the subject at the end.
Notes: related issues are Enhancement #137955, which requests ability to customize subject/title of the transaction, the other issue for this concern is #164332.