Please let merchants integrate custom SMTP like sendgrid, AWS SES, mailgun, etc for their custom domains.
Currently all email notifications are sent under the sellix domain. Let's say a user had bad experience with a random seller on sellix and they marked previous emails from sellix as spam, now this will affect email notifications sent for my store too.
If I am able to use my own custom SMTP or transactional email provider for my domain, I don't have to worry about it.