Are these toothbrush heads compatible with the io6?

Yes they are compatible with the IO range (I have the IO8)

What is the difference between the mailbox and regular?

The mail box version is small enough to fit in your letter box. Your Oral-B Team

Will this head fit iO 6?

Yep. As it says in the description "Oral-B iO brush heads are exclusively designed to fit Oral-B iO toothbrushes" so if your Toothbrush is an iO then it should fit

Are these compatible with Oral B Vitality toothbrush recently bought please?

These are not compatible with Vitality toothbrush. The iO toothbrush heads will only with with iO handled. All other toothbrushes except iO and Pulsonic will fit with your Vitality toothbrush. Your Oral-B Team

Product Name	Oral-B Toothbrush
Weight	0.12 Kilograms
Units	4 Count
Manufacturer contact	Procter & Gamble UK, Weybridge, Surrey, KT13 0XP
Country of origin	Poland
Brand	Oral-B
Format	Sensi White
Age Range Description	Adult
Country of origin	Germany

Oral-B Gentle Care

Details

Description

Specifications

Common Questions