When using Smtp.Send(filename) the file stays locked after sending the mail. Send(stream) with a FileStream works as a workaround, but it seems that you do not close the file/stream when sending by filename.
UPDATE The fix has been released as a part of version 2014R2.
This was a bug in the component. Sorry for inconvenience! It's surprising no one noticed until now. It has been fixed for the next release. Until then, a hotfix is available (contact us if you need it).
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