Hello @ReginaS5 ,
As long as your branding is still being used for the other emails not related to sales receipts, then you should be okay as long as your contacts have granted your company permission to email them. If the 3rd party will be using any of their branding over yours in the regular advertisement, newsletter, etc. emails then you may need to go through a Confirm Opt-In process for them.
Ultimately, it'd be best to reach out to our Compliance / Review teams so they can provide some more specialized insight, to make sure you're 100% covered on this transition, and that the rights of your contacts are protected.
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