An email opt-in is not a texting opt-in, full stop. The person agreed to receive emails; they didn't agree to texts, and texting is held to a higher consent standard. Treating your email list as a texting list is one of the most common and most expensive mistakes campaigns make.
Why email consent doesn't transfer
Consent is channel-specific and purpose-specific. Someone who typed their email into a petition agreed to emails about that cause, not to text messages, which carry their own prior-express-written-consent requirement. Courts and regulators treat the two separately.
How to convert an email list the right way
Ask. Email your list with a clear invitation to opt in to texts (a text-to-join keyword, or a link to a form with the proper disclosures). The people who say yes become a clean, consented texting list. The rest stay on email, where they already agreed to be.
Frequently asked questions
Can I text people who gave me their email?
No, email consent doesn't equal texting consent. You need separate, documented prior express written consent to text them, even if they opted in to your emails.
How do I turn my email list into a texting list?
Invite them to opt in: email a text-to-join keyword or a form with proper disclosures. Those who affirmatively opt in become a clean texting list; the rest remain email-only.
Informational, not legal advice.
Keep reading: What is prior express written consent for texts and Can I text people who signed my petition.