The issue might be caused by low-quality contacts with a lot of outdated or incorrect emails that can hurt your domain reputation and negatively affect your future sequences. We strongly recommend that you validate your contact lists before launching a sequence. You may use our validation service or any other service you prefer:
Please also note that in Reply we have a minimum requirement for an acceptable email bounce rate. If a new sequence reaches a 15% bounce rate, such a sequence stops sending the first step to contacts but continues sending follow-ups. We will notify you of sequences that have reached half or full bounce quota so that you can prevent sequence suspension.
For more details on how to improve your overall email deliverability score, please refer to this article.