This could be caused by various things,
By the size of the mailshot being created:
If it is being sent to a large number of recipients please try splitting it into smaller batches.
If it has lots of large images or attachments, please compress the images or reduce the number/size of the attachments.
The computer running RDB Pronet running out of resources, please reduce the number of programs/tabs open and try again.
Problems with the template/merge fields in the template being used, please try a different template or try removing and re-adding the merge fields in the template being used.
If none of this helps, please contact us via the MyAccess portal and reference the title of this article, the name of the mailshot that this is happening for and the name of the user affected.