SMTP Error 550 generally comes with a side-message like "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site".
This type of error code generally means that your SMTP Server is unable to deliver the communication to the intended recipient/s because his/her mailbox/address does not exist. In most situations like this, either the email address was entered wrong (for example: firstname.lastname@example.org instead of email@example.com), or the account has been deactivated and replaced with a new one (but you didn't know it).
In any case, you would need to update your mailing list and constituent records with an accurate email address for the intended recipient and/or remove the existing one from the list. If you leave the invalid email address in your list you will likely continue to receive the 550 error messages.
Note: If you continue to attempt to send to the email address/es that are returning this specific 550 error code, you could be penalized and see your delivery rate decrease, as your organization/IP Address may be flag as a SPAM address.