When you say 'cannot send' do you mean:

• Thunderbird does not recognise as valid email address and so the 'Send' button does not get enabled.
• You click on 'Send' and then it seems to get stuck and eventually fails?
• Do you get an error? If yes, what is the error message word for word.
• You send ok but then recieve a mailor-daemon email saying it cannot deliver? If yes, then please post an image showing contents of that email.

Look, let's not make this complicated. I can send emails to any email address except the one that has dash-mark(-) in it. Here, I'll give you the email address: [email]@tlb-cpa.com A support person I spoke with at RR said that Thunderbird is not recognizing the dash-mark(-) Regards, John

I don't have this issue for information (my own email à a dash inside and I still receive a loooot of email from my client using TB) Maybe thiese email à wrongly record in your adress book ? Have you try to send an email to a fresh dash email ? Or to delete these email from your adress book and then re-try ?