get https://api.sendios.io/v1/unsub/unsubreason/
API endpoint to return the unsubscribe reason.
There are the following reasons for unsubscribing:
- settings_all_types – the user unsubscribed from the mailing list in the product settings;
 - fbl – the user complained about the mailing in the mailer;
 - admin – the user was unsubscribed through the admin panel;
 - client – the user was unsubscribed in your system (or his account was deleted from the product);
 - header – the user unsubscribed from the mailing list via the mailer using the email header;
 - invalid – invalid email address;
 - settings_oneclick – complete user unsubscribe from email in one click;
 - page – user unsubscribed via Sendios page;
 - mx_invalid – the user was unsubscribed due to an invalid mail domain;
 - distance_bounce – the user was unsubscribed due to a non-existent email address.