Email Subject field character limit in Salesforce User Interface and API

Character limit for email subject field in Classic UI

There is the 100 character limit on the Email Subject field when using the Send an Email button (Email Author) within the Classic UI. 

NOTE: In the Winter '16 release, a new feature was added that allows for a maximum of 650 characters in email subject line to be displayed when using the Email Publisher action in Case Feed only. The limit of 100 characters on the Send an Email button (Email Author) still applies.
 

Character limit for email subject field in Lightning Experience (LEX) UI

User can type a maximum of 3000 characters on the Subject field when using Lightning interface using the Email global publisher action or Object specific 'Send Email' quick actions.
 

Character limit for email subject field in Email Template

There is the 230 character limit on the Email Subject field within Email Templates when using Classic UI or Lightning UI.

NOTE: Email Templates will have hardcoded limit of 230 characters on Subject field even if they are created from UI or through API.
 

Character limit for email subject via API

The subject field on Email Message object 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; however, outbound emails from within Salesforce are limited to above specified character limits depending on the UI they are drafted/composed.

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 emails with a large Subject line have the value 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. For example: Google or Gmail allows typing more than 4000 characters when you draft email, but truncates to 998 characters in subject line when its sent.

While there is no mention of character limits, we suggest that you read our Developer Documentation to learn more about this field.