Debbie, sometimes Amazon will temporarily remove your privileges to send ‘proactive’ emails if they think you’ve broken the rules somehow. If so, they will send you an email to explain that, and also the emails shown in the SENT box of your sellercentral Messages will show a Red message explaining why.
Issues with emails almost always relates to content, and rarely, if ever, relates to the email service you’re using. I use Helium10 for emails with no problems and I suggest that H10 is not the cause of the issue here.
To narrow down the issue . . .
1. Did you receive an email from Amazon regarding a temporary restriction on sending emails?
2. What is the exact wording of the Red message that you refer to? (Please cut and paste or screenshot it) Also, did you see the red message in Sellercentral or in Helium10?
3. What are the statuses of the emails according to Helium10, e.g. Sent, Opened, Blacklisted, Opted-out, etc. ?
4. Are you including any attachments or URLs in your emails?
5. What was the ‘generic response’ that you received from Amazon (and what did you ask them?)