A lookup is always a 1 on 1 relation, so this is not possible. Is there a limited set of options they can select or can they select all values? If so, how many options are there to choose from?

They can select all values linked to that item. In this case there are attachments linked to an email that the user can select. The user can select 1 attachment and it will be added to a list of attachments that the user is going the send.

I want to select multiple attachments so the list can be completed at once, instead of selecting the attachments one by one in the lookup. 

Hi,

It would be more handy to do this in either a separate screen or with Task logic in combination with multirow task execution. Like Erwin said, lookups are meant for a 1-on-1 relation and cannot be used for this.

Thanks. I have now created a new screen indeed for this.