I just did a little more research, and it appears that Express Invoice for Linux wasn't even linked against openSSL during its build. Unless I'm missing something, I have no hope of emailing via GMail with the Linux version of Express Invoice:
jshusko@jshusko-laptop:/opt/nch/expressinvoice/bin$ ldd expressinvoice
linux-gate.so.1 => (0xb7fa6000)
libgtk-x11-2.0.so.0 => /usr/lib/libgtk-x11-2.0.so.0 (0xb7be6000)
libgdk-x11-2.0.so.0 => /usr/lib/libgdk-x11-2.0.so.0 (0xb7b59000)
libatk-1.0.so.0 => /usr/lib/libatk-1.0.so.0 (0xb7b3d000)
libgdk_pixbuf-2.0.so.0 => /usr/lib/libgdk_pixbuf-2.0.so.0 (0xb7b23000)
libpangocairo-1.0.so.0 => /usr/lib/libpangocairo-1.0.so.0 (0xb7b17000)
libpango-1.0.so.0 => /usr/lib/libpango-1.0.so.0 (0xb7ad4000)
libcairo.so.2 => /usr/lib/libcairo.so.2 (0xb7a5a000)
libgobject-2.0.so.0 => /usr/lib/libgobject-2.0.so.0 (0xb7a1b000)
libgmodule-2.0.so.0 => /usr/lib/libgmodule-2.0.so.0 (0xb7a16000)
libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0xb7a12000)
libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0xb795a000)
libasound.so.2 => /usr/lib/libasound.so.2 (0xb7892000)
libresolv.so.2 => /lib/tls/i686/cmov/libresolv.so.2 (0xb787b000)
libgthread-2.0.so.0 => /usr/lib/libgthread-2.0.so.0 (0xb7875000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xb7786000)
libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb7760000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xb7751000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb75ee000)
libX11.so.6 => /usr/lib/libX11.so.6 (0xb74fe000)
libpthread.so.0 => /lib/tls/i686/cmov/libpthread.so.0 (0xb74e5000)
libXext.so.6 => /usr/lib/libXext.so.6 (0xb74d5000)
libXrender.so.1 => /usr/lib/libXrender.so.1 (0xb74cb000)
libXinerama.so.1 => /usr/lib/libXinerama.so.1 (0xb74c8000)
libXi.so.6 => /usr/lib/libXi.so.6 (0xb74bd000)
libXrandr.so.2 => /usr/lib/libXrandr.so.2 (0xb74b5000)
libXcursor.so.1 => /usr/lib/libXcursor.so.1 (0xb74ac000)
libXcomposite.so.1 => /usr/lib/libXcomposite.so.1 (0xb74a8000)
libXdamage.so.1 => /usr/lib/libXdamage.so.1 (0xb74a5000)
libXfixes.so.3 => /usr/lib/libXfixes.so.3 (0xb74a0000)
libgio-2.0.so.0 => /usr/lib/libgio-2.0.so.0 (0xb7431000)
libpangoft2-1.0.so.0 => /usr/lib/libpangoft2-1.0.so.0 (0xb7408000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0xb7391000)
libz.so.1 => /lib/libz.so.1 (0xb737b000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0xb734e000)
libpixman-1.so.0 => /usr/lib/libpixman-1.so.0 (0xb730a000)
libdirectfb-1.0.so.0 => /usr/lib/libdirectfb-1.0.so.0 (0xb72a4000)
libfusion-1.0.so.0 => /usr/lib/libfusion-1.0.so.0 (0xb729b000)
libdirect-1.0.so.0 => /usr/lib/libdirect-1.0.so.0 (0xb7286000)
libpng12.so.0 => /usr/lib/libpng12.so.0 (0xb7260000)
libxcb-render-util.so.0 => /usr/lib/libxcb-render-util.so.0 (0xb725a000)
libxcb-render.so.0 => /usr/lib/libxcb-render.so.0 (0xb7252000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0xb7238000)
libpcre.so.3 => /lib/libpcre.so.3 (0xb7206000)
/lib/ld-linux.so.2 (0xb7fa7000)
librt.so.1 => /lib/tls/i686/cmov/librt.so.1 (0xb71fd000)
libXau.so.6 => /usr/lib/libXau.so.6 (0xb71f8000)
libselinux.so.1 => /lib/libselinux.so.1 (0xb71de000)
libexpat.so.1 => /usr/lib/libexpat.so.1 (0xb71b7000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0xb71b2000)