When an email is sent to a May First member, it first enters the spam filtering service in our mail server. That server is looking for a set of characteristics that would indicate that the mail being sent is fraudulent. Should the email score too many “points” the email will be marked as spam, and moved to the spam folder when it is received by the mail box.
Our server side spam filtering isn’t the only form of spam filtering that is available. Round cube out of the box doesn’t perform any spam filtering, however users can add their own filters. If a member is seeing spam, if any of the spam emails share a common characteristic, members can set spam filters to move those emails directly to the spam folder.
In addition to that third party clients such as Thunderbird and Mac mail can perform their own spam filtering. If you want algorithmic spam filtering, where when you click on the spam button that information is used to determine what gets filtered out in the future, a third party client is currently the only way to achieve that. For an example of how this works, here is Thunderbird’s documentation on spam filtering.