Please post a screenshot of the error message. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

I followed the link but apparently I'm too inept to make a screen copy but here's what the message says:

L’envoi du message a échoué. Une erreur est survenue lors de l’envoi de l’e-mail. Le serveur a répondu : Reject for policy reason RULE1_1: Virus/Malware detected. Veuillez vérifier le message et essayer à nouveau.

What do you mean by 'the send link'? Is there a reference to a site in your message? Can you send a simple plain text message? Do you use a sig file that has a link?

I don't have a problem when I send ordinary emails, even those with attachments and/or links within the email. My only problem arises when I use the Mozilla large file transfer service "Mozilla Send". "Mozilla Send" stores the file and sends me a link that I send to a third party or multiple third parties so that they can download the file. It is when I try to send this link that I get the error message. I'll try again to get a screen print.

Ah, I got the screenshots of the link and the error message, see attachments

Just to clarify the terminology: You're using the Thunderbird feature Filelink, which requires a Filelink storage service. You set up your Filelink to use the Send service instance (formerly Firefox Send) send.vis.ee as a storage service. Presumably you also need an add-on for this (https://addons.thunderbird.net/en-us/thunderbird/addon/filelink-provider-for-send/ ). Note, the Firefox Send service was discontinued by Mozilla in 2020.

The error message you get is from your email provider's server. Apparently they believe your message contains malware, or that https://send.vis.ee is a malicious site, but obviously it isn't. So this is a false positive on their end. It's not your fault, but unfortunately there is little you can do.

• Try to talk to your email provider, and explain to them that https://send.vis.ee is a legitimate web site.
• If you compose your messages in HTML try plain text instead.
• Try a different Filelink storage provider with a supposedly better reputation, e.g. Dropbox (https://addons.thunderbird.net/en-us/thunderbird/addon/filelink-provider-for-dropbox/ )

I've run my anti-virus (Avast Premium)

Most likely unrelated, but Avast is known to have caused problems in connection with Thunderbird in the past.

These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

OK, I'll send a copy of your response (christ1) to my email server's support service. Many thanks to both you and david for your interventions. Happy New Year!