One reason individual Emails may not be getting to your contacts could be the size of the attachments. The maximum attachment size is 12MB. Any Emails with attachments over this amount may or may not be received. You will not receive any error notification when this happens.
Note: This is not a VanillaSoft issue.
Email standards such as MIME don't specify any file size limits, but in practice email users will find that they can't send very large files.
Over the Internet a message will often pass through several mail transfer agents to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore need to impose size limits. The result is that while large attachments may succeed internally within a company or organization, they are unreliable when sending across the Internet – and for that reason sending systems often arbitrarily limit the size their users are allowed to submit. As an example, when Google's gmail service increased its arbitrary limit to 25MB it warned that: "you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits". In general, 10MB is considered safe for the maximum size of an email.
Email users can be puzzled by these limits because the MIME encoding adds 33% overhead – so that a 20MB document on disk exceeds a 25MB file attachment limit.