A "bounce" generally occurs when sending emails to addresses that: do not exist/spelled incorrectly, have full inboxes, the email address is too old and not used anymore, block domains with poor sender reputation (too many spam complaints), specific content in your message is flagged, and/or a restrictive DMARC record for your sending domain. It is recommended to keep your "Bounce Rate" as low as possible as this becomes a problem if it gets too high can lead to a disabled domain.

Common options for reducing bounce rate:

Clean Your Mailing List

Recommended
Delete Email Addresses that are invalid, inactive or obtained without specific consent to receive emails.

Authenticate Emails

Recommended
Verify the email address provided to you is correct. This usually involves sending a confirmation email to the address provided by your user and having the user click a link to verify the email is valid. This prevents the user from providing an incorrect email or email they do not use/own. Once validated, it is then best to pass to OneSignal.

Email Consent Flow

Recommended
For best deliverability, you should only send emails to recipients that have knowingly signed up to receive emails. When a person uses a valid email to sign in to the app, they don't necessarily acknowledge that they want emails from you. If you use a prompt asking the customer what kind of emails they want, you can then set tags on the user marking what kind to send.

Email Validation & Suppressions

If you already have a list of emails and want to validate them, Mailgun and SendGrid provide Email Validation tools that you can use to check a list before uploading to OneSignal. See:

• Mailgun Email Validation API
• SendGrid Email Validation API

Once you have a validated list, you can import emails following the Import Email Addresses guide.

If you already sent emails to a list, then your ESP likely has a Suppression List. Usually any emails you target through OneSignal that are on the Suppression List in your ESP will not be sent and will get marked as unsubscribed in OneSignal.