RAT infection is the most scary reason, but there are many more reasons your email could be on the street. In terms of increasing probability: a data leak at some company who has your email or you entered your email to download something or use free WIFI, or you responded to a discount or prize winning action, or a friend has installed an app in his/her phone allowing access permission to all your friend's contacts, etc ...
I once downloaded a free app (from the Google store) which function was to check whether something is completely horizontal. When I read the permissions it asked access to my contacts and pictures (why ???) and I immediately removed it (with netGuard temporarely enabled on my phone it could not have gone outbound).