But i don't know where they know my email
~
i always send my email in Airdrop form.
you entered your email address in every airdrop form you filled out
your address and other info are being sold or shared to other airdrops, ico projects, etc
I never join that channel and maybe this channel is from fail or fake airdrop
you can change your telegram settings to prevent that from happening again
go to Settings > Privacy and Security > Privacy:Groups > My Contacts , apply changes
that would make only known people in your contacts can add you to a group/channel