How does it handle syncing contacts that are not in Salesforce?

Swift Digital (SD) subscribers and Salesforce (SF) are connected via shared Mailgroups. You are able to create mailgroups at SD and share it with SF. Alternatively, you can create a mailgroup from salesforce

If you add a member to a shared mailgroup it will appear both in SD and SF.

When sharing a mailgroup from SD to SF for the first time, all members of the mailgroup will arrive as leads. This is a SF restriction.

For this reason a company field must have a value, as SF will reject leads without company field.

Example: If you have a member of a shared mail group that does not exist in SF, this member will be sync'd and arrive as a lead.

Some customers use a convert to contact application (such as Dedupe Manager, a market place app) if they wish to convert leads to contacts , and then if a contact already exists it can also de-duplicate it without losing the SD Person ID.