What you're describing may be a limitation of the fairly simple converter Word for Mac uses for mailmerges or a limitation of merging to email on that platform. If you merge to a new document, inspecting the output should be sufficient to let you know where the problem occurs. If the dashes reproduce correctly when merged to a document, the problem is in the email conversion; otherwise it's a mailmerge limitation. Either way, though, you may need to use something different (e.g. single & double standard hyphens in the data source) to simulate the required output.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]