I would just do this manually.
Export your photos to a folder of your choice.
Start your email program.
Create the new email message; attach your exported photos.
That said, email is not a good way to send large quantities of data.
Although I am using Windows 7 I am also using Thunderbird as an email client. It does appear in the list for emailing images from LR.
First restart your Mac and test.
Second reinstall TB over the top of the current install of TB.
It seems LR is not seeing it as an install Email client and a reinstall may help register it as such.
Is any email client showing up? Like Apple Mac Mail
Thank you for your answer. What you suggest was the only solution left for me to send me photos with email. So did I.
But, as the option exists in LR and requires less clicks than doing it manually, I wanted to fix it.
And I agree with your opinion on sending heavy attachments: not a good idea! I use email only when I want to send a few photos.
Thank you for your answer.
I restarted my Mac, and nothing changed.
LR doesn't show any email client.
I can't reinstall TB now because I have no backup these days I'll try it later.
IIRC if you reinstall TB over the top of the current install it should not delete or remove any of your emails. TB stores emails in a different folder, usually in your User Name folder. It is easy to find where they are stored. In the Tools menu item under Accounts go to the sever Settings and at the bottom of that page it lists Local directory. The path to where TB stores all your email is listed there.
Either Macintosh HD or Home (Home is what Apple calls the Username folder).
You can copy the Profiles folder to some other location just in case.