Some mail client programs does not allow FileInMail sending messages through them without showing a security alert message.
How to disable that message to send mail without user intervention depends by the specific mail client used. For example:
Microsoft Outlook Express:
Go to Tools -> Options -> Security
Remove the checkmark near the "Warn me when other applications try to send mail as me" option
Mozilla Thunderbird: The first time Thunderbird is invoked by FileInMail it
shows you the warning and asks you if you want to see it again in
future. Answering "no" will setup the mapi.blind-send.warn parameter as
false. To change it programmatically:
Go to Tools -> Options -> Advanced -> Configuration Editor
Setup the "mapi.blind-send.warn" option to false (to disable the warning) or true (to enable it)