Outgoing SMTP issue with large attachments

Friends,

Using the SNDDST command to send spool files to email addresses, I'm
running into the following situation:

- emails with relatively large or small attachments are sent effectively
to external email addresses.

- emails with relatively small attachments are sent effectively to
internal email addresses.

- emails with relatively large attachments disappear without a
noticeable trace, allow the SNDDST command executes without error.

I do not have ready access to the system.  It is the discrepancy in
behavior between the large and small attachments which I find
perplexing.

I would appreciate your thoughts,
Regards,
Andy Nolen-Parkhouse