Which activity allows customizing the sender's name in an email?

The option that allows for customizing the sender's name in an email is the activity associated with sending Exchange mail messages. This activity leverages Exchange Web Services (EWS) or Microsoft Exchange Server capabilities, allowing for more control over various email attributes.

In contrast, other options such as sending SMTP mail messages may not provide a straightforward way to modify the sender's name, as they rely on the credentials of the SMTP server being used, which are typically fixed. The same applies to sending mail notifications, which usually adhere to default configurations set within the email system or the user profile settings. Although sending mail messages covers a broad range of mail types, it generally does not include advanced customization options like the Exchange mail activity does.

Thus, the ability to manage and customize email attributes such as the sender's name is a significant feature of the Exchange mail message activity, making it the correct choice for this question.