I just found out the hard way that many email providers, such as gmail, drop tribe's emails if you use a custom domain. The reason is all emails are sent with tribe's sendgrid account, which only has the correct DKIM and SPF settings for emails originated from tribe.so. So if you use a custom domain, you must change the setting so that the "from:" email address is from the non-custom domain, ie: no-reply@XXX.tribe.so