Version: 5.4.0.3
tested on Win7 SP1, deb8
In ODT documents containing horizontal lines, the lines are not rendered when sending as PDF attachments to e-mail recipients. Changing the thickness of the line makes no difference.
Saving the ODT as a PDF with File->Save As works as expected.
Is a different PDF rendering library used in each case ? If so, the Mail Merge one is broken.