Sending transactional emails from same address as marketing emails: yes or no?
Marketing emails require opt in according to GDPR, transactional emails don't.
Is it okay to use your existing marketing platform to send transactional emails eg registration confirmation email, knowing they require opt in and user may mark marketing emails as spam resulting in non delivery of transactional emails?
No, if you can avoid it. Ideally these are two separate email addresses with two separate subscriber lists
Can be a real headache keeping these tied together.
Ah yes, good point. Lists should be kept separate.