If you host seminars, workshops or attend trade shows, these are great venues for collecting email addresses to build your list. Recently, there was a question on Linkedin about the proper use of double opt-in in such cases.
Perhaps in this case double opt-in is not required. Attendees will probably be given a form to fill out at the event. This form should clearly state that by giving their email address, they will be added to a list named “XYZ” owned by “Name”
If you choose double opt-in (also called confirmed opt-in), then make sure the FROM address is readily recognizable to the recipients, and that the subject line includes a reference to the event. Don’t use the typical subject line: “[firstname] please confirm your subscription,” instead, try something like: “[event-name]: your free bonus material” and then deliver the bonus material on the confirmation “thank you” page or by delivering it via auto-responder once the recipient confirms the subscription.
Don’t be afraid to use the word “free” and certainly do not try to obfuscate it like this: fre-e, f.r.e.e as that will certainly trigger spam filters more so than using the word in the open.
One key element of any successful email marketing campaign is content. If the follow up emails contain only advertising for your books, courses, etc. then most of the recipients will mark the emails as spam — even if you use double opt-in. It’s all about delivering what’s expected: if you say “monthly enewsletter on subject” then don’t send 27 other messages in a month that are purely promotional.