Don't do it like that: it's probably in breach of GDPR regulations, where an email address is "personal information" and must be safeguarded under penalty of huge fines.
When you use multiple "To" addresses, you send the email address of everyone in the collection to everyone in the list: all recipients can read everybody else's email address. That's a bad practice, and can literally bankrupt even large companies.
Instead, either send individual messages (slow and painful) or send it to a "dummy address" and add all recipients as BCC (Blind Carbon Copy) addresses. Either way ensures data protection.
In your code however, you attempt to add each address twice: once in the loop, and a second time immediately after. The latter is what is causing your compilation problem: there is no overload of the Add method which accepts an array of strings.
If you look at the documentation, the Add method accepts a single sting which can contain multiple recipients separated by commas anyway ... so your whole "Split and loop" code is redundant ...
MailAddressCollection.Add(String) Method (System.Net.Mail) | Microsoft Docs[
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