A bit off-topic: problems with sending to a Gmail user

[Ofer Inbar]

> PS: maybe there is an option in gmail for users to whitelist senders
> from whom they want to receive mail?

Yes, sort of.  There is no absolute whitelist, but there are two
things a gmail user can do to make it much less likely that mail
from a particular sender is marked as spam.

1. Have that sender in their contacts.

2. After receiving an email from that sender that does go to spam,
click "not spam" on it.  Sometimes you can arrange this by private
messaging with someone as you send them email, and have them check
spam for the email you just sent and click "not spam" while you're
talking.
  -- Cos