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>Maybe I should send a message from my barcode application to some sort of queue table, and have a separate application running that does the email sending, so that control returns to my application right away?
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I would go with a queue and separate email application:

1. Your application wouldn't be slowed down or locked up by email part
2. If you've multiple application running than you don't have to mainatain email part on each of them.
3. It's easier to change emailing application because it wouldn't affect the rest of your application.
4. You can implement some other ways of delivering messages besides emailing them if you've to w/o affecting the rest of your application.