It's hard to see what, if anything, you are doing wrong or could do differently. I'd look at your recipients' email providers' configuration to see if they are blocking or stripping incoming attachments.

Anandk said

Recipients of mails always complain of missing attachments. I have "missing attachments" add on installed and do not miss any attachments. My sent mails show the attachments, but the same are not delivered/ reported as not delivered by the recipients. The attachments are not too big (less than 1 mb) and do not contain any exe or such prohibited content. If I mark a copy of the mail to myself, I can see the attachment in the received mail in thunderbird.

No solution to the problem? Feeling frustrated.

As Zenos said.' said

It's hard to see what, if anything, you are doing wrong or could do differently. I'd look at your recipients' email providers' configuration to see if they are blocking or stripping incoming attachments.

My guess is the recipients mail server is stripping the attachments. Probably because they are not an "approved type" only this week I had someone trying to send me an email with and EML attachment. Google bounced the message. Apparently now your not allowed to forward a message as an attachment.

The attachments are excel, word or pdf files - which are not prohibited, I suppose. The customers are getting attachments from other senders or even sent by me from browser (without using thunderbird). After trying all possible solutions, I could come to the conclusion that there is something wrong with Thunderbird or its configuration. Can some one help, please?

Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.