Are you on the latest version of SI? That would be 2020.4.7. I’ve run several tests on it and seems to work OK for me. I will look more into the swiftmailer package that is used for email to see if the error you are porting is a known issue.
I should have looked up the 3.6.2 section first. I assumed this was the “to” address that you were trying to put multiple addresses in. However the error deals with the email origination fields; from, sender and reply to. Of these only the “from” field is set. If you read the help on the from field, it says specifically that it does not support multiple email addresses. Basically and by default, the from field is intended to be the biller’s email and gets loaded with the biller’s name for the friendly part of the address. Does this address your issue?