The SwiftMailer replacement, Drupal Symfony Mailer → , isn't supported.
Enable the module and observe that sending mail with Views Sends fails with a "TypeError: Symfony\Component\Mime\Header\UnstructuredHeader ..." error message. (If you send directly using batch API, you'll observe that sending hangs with "Initializing...".)
In addition, MIME (HMTL formatted e-mail / attachments) support isn't activated, because Views Send doesn't know about this module at all.
Add support Drupal Symfony Mailer. Wait for a fix for 🐛 MailManagerReplacement::mail() doesn't always compose Active .
Active
2.0
Code
I could use this too. If you let me know when you have a first patch I'm happy to test it.
Adding some related issues in the Symfony Mailer project that I think needs to be resolved. Normal MIME / HTML mail should work as soon as those issues are resolved (or a work-around presented). I haven't started looking at attachment handling, but I expect that to be trivial changes - if any.