Have you tried to save the attachment and then re-attach it when you forward? It may be that the sender or his mail client did something that doesn't relay well, but by rebuilding the message you might improve matters.

You don't say why or how your recipients "…cannot send the same email…". What happens? What errors or error messages do they see?

I don't think they get an error message they just don't get the attachment. I get a lot of photos to forward and then those people have to forward them. It would be a lot of bother to save each photo. Thanx for your reply

Zenos said

Have you tried to save the attachment and then re-attach it when you forward? It may be that the sender or his mail client did something that doesn't relay well, but by rebuilding the message you might improve matters. You don't say why or how your recipients "…cannot send the same email…". What happens? What errors or error messages do they see?

I have tried this but it still doesn't work. So many people cannot open .eml files. If I send an E-Mail by forwarding they do not get it the attachment.

EML is industry standard mail files. even gmail can open them. Pull the other one please.

If Thunderbird sends the mail and the recipient can not forward, what do you think anyone here can do? To the best of our knowledge Thunderbird sends RFC compliant emails. The recipient will need to take up the failure with their Mail reader provider to find out what the issue is.