Attachments </= 40k sent from POP do not appear in iNotes

I have run into a strange issue where attachments that are </= 40k sent to an iNotes user are not detectable in the iNotes client, but if that same message is viewed from a full Notes client, the attachment is visible. Attachments >/= 41k are visible in iNotes.

The iNotes client is using an R5 template and the mail db is on a 5.0.11 Server on AIX.

The messages in question are being sent from POP (Thunderbird) users, who access their email via Citrix. The Thunderbird users’ mail dbs (R5 & R6 templates) are all on R6.5.1 servers on AIX.

Messages with attachments 40K or smaller sent from other email clients to the iNotes users on the 5.0.11 server are fine.

Messages with attachments 40K or smaller sent from Thunderbird to iNotes users on the 6.5.1 server are also fine.

Do you have any idea what is causing this problem and how to fix it?