SMTP addresses aren't as simple to deal with as they seem in your example.
For example, it's possible (although it's no longer used very often) to have a space (or other "special character") in the "local-part" of the address that requires quoting (e.g., "Full Name"@domain.edu).
Is your string using space or tab characters to separate the addresses?
Are these addresses coming from a MS Exchange object in the AD? If so, why not use the property "PrimarySMTPAddress" from the mail/mailbox-enabled user/contact/group/etc.? That makes things a lot easier.
If you're dealing with AD objects and their proxy addresses, you can follow this article:
https://itfordummies.net/2015/06/17/identify-primary-smtp-address-from-active-directory-with-powershell/
--- Rich Matheisen MCSE&I, Exchange Ex-MVP (16 years)
