I call Thunderbird from a program to send out emails in batches. When an attachment exceeds approximately 14 MB, Thunderbird stops sending and requires manual intervention. The messages become stacked in a queue. I then must click the "Write" button for each message. As I do this I get a warning message dialog advising that the message exceeds 20 MB and I must confirm. Once I confirm, the message will send normally.

How do I avoid this requirement to address the warning dialog?