Hello @HaleyS1 ,
As mentioned in my reply from 2020, you can check out our Partner program to see if that would fulfill your needs of being able to have your own account and work within your clients' accounts. Otherwise, if your plan is to have just one account for multiple companies, the first half of my response still applies - it's technically possible, but frankly not recommended or easily maintained.
Additionally, with the 2024 changes to Google and Yahoo's authentication requirements, it will also negatively affect the delivery of emails overall to be trying to send as multiple separate companies under a single account.
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