Why is the Email Subject character limit different in API vs UI?
|Knowledge Article Number||000176857|
|Description||There is the 100 character limit on the Email Subject field within the UI. Note: Starting Winter'16 we have added a new feature where a maximum of 650 characters in email subject line is displayed using the Email Publisher in Case Feed only. The limit of 100 characters on the Send an Email task (Email Author) still applies.
However, the subject field can store 4000 characters in the database. If the data is entered via the API (Email-to-Case, Data Loader, Integrations, etc.), more than 100 characters can be entered, and will be stored in the database. The UI will actually display more than 100 characters if the Subject is longer than 100 characters, as long as the email was created via the API (not limited by UI constraints); however, outbound emails from within Salesforce are limited to 100 characters.
The Email Subject field has a hybrid length in order to maximize compatibility, both inbound and outbound. Some email platforms allow senders to enter more than 100 characters, which is why the field allows inbound email to store thousands of characters, to ensure that if an email is received with a large Subject line, the value will be retained. Other email platforms may truncate subjects longer than 100 characters, or even 50 characters. We limit the Subject length for outbound email in order to avoid truncation as much as possible once the email reaches the intended recipient.
This document explains the field, but does not mention any specifics about length: http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_objects_emailmessage.htm#topic-title
This discussion may also be helpful in explaining the reasons: https://success.salesforce.com/questionDetail?qid=a1X30000000JGHiEAO