The emails I am sending out via convertkit are getting caught in spam-filters although I am very careful with my wording etc. My email-provider says it is because the emails are being sent via converkit's server and says that I should ask convertkit what needs to be added to the "SPF record"? Then he will do that and things should work better. As you can sense, I do not fully understand this, but can you please let me know: what needs to be added to the SPF record so the chances of my emails getting through is improved?