First, I am NOT the internet guru, so you may receive more detailed info from others. The DKIM message indicates that your email id and the id of the server are a mismatch. This often happens when using self-hosted email and the server is not properly configured. This is not something you can do in Thunderbird, but is a server setting to be done by whoever owns the server. The recipient's email host has security in place that is rejecting your message.

Hi David, thanks for your reply. I have checked with the hosting company and they say that all is well. My emails are not bounced if I use webmail only when I use Thunderbird. It seems as if the problem is originating with Thunderbird or the way I am using it! I haven't changed any settings and have used TB for over 10 years and haven't had this problem before so I'm a bit stumped.

I respect that webmail works, but DKIM is different. This is not a thunderbird issue. This URL may help:

https://bobcares.com/blog/resolved-550-dkim-verification-error/   or this one 
https://myemailverifier.com/blog/550-dkim-signature-verification-failed/

Hi David,

Thanks for your reply. I am looking through those links now and hoping to find something I can pass on to my hosts greengeeks to see if they can rectify the issue.

I don't understand how it can be that webmail doesn't bounce and email sent via Thunderbird does bounce and that Thunderbird isn't the issue. If there were something wrong with the sending setup at the host server would webmail not bounce as well? I'm afraid I know very little about how this works, but it just doesn't seem logical that webmail works yet the server has set up issues.

Thanks again!

Webmail uses a different setup. Your host knows all about this, at least at their senior support level.

Hi,

I followed the links that you suggested and used the DKIM tester at https://dkimvalidator.com/. I cound not found any issues found in the report and forwarded it on to GreenGeeks support to see what they might find. Here is their reply:

According to the test you submitted at https://dkimvalidator.com there is no issues related to workingdata.co.uk DKIM.

We have also tested the DKIM for workingdata.co.uk using the https://www.mail-tester.com/ service and it didn't detect any problems:

At their suggestion I added my email account to MS Outlook and was able to send email to the domain which was being bounced from Thunderbird.

I also uninstalled Thunderbird v128.8.1 esr and installed v137.0 and emails are still being bounced with the same error as before: 550 DKIM Sender Invalid - envelope rejected. I have removed all add-ons and have just the default themese installed.

I'm afraid I am completely stumped.

Hello,

I have sinced updated to v138.0 and the issue is still there. GreenGeeks support suggested trying with line wrapping turned off and sending as plain text and this did not help. They themselves are able to send emails from my account to the recipient domain without issue.

They say that the antivirus software I am using may be interfering. I am using Norton Security, is that a possibility?

Any suggestions would be gratefully received!

Many thanks.

What outgoing server are you actually using to send your mail. sure the DKIM is checking out for workingdata.co.uk. But that will not help if the outgoing server being used is say the one for say your internet service provider. Given you are using the troubleome product from Norton we can expect more than the usual issue what with their auto enabling VPN which does bad things with email and it's use of self signed certificates requiring two exception in your mail client before you get any mail at all.

The instruction from greengeeks.com for Thunderbird are more than a decade old. https://www.greengeeks.com/tutorials/how-to-use-mozilla-thunderbird-3-for-email/ but something do not change much.

SO can you please post the account settings you are using.

1. Open the menu > Help > Troubleshooting Information, then click Copy text to Clipboard.
2. Go to https://pastebin.mozilla.org/, paste the clipboard by right clicking in the large text area, select paste from the menu, change the retention period to expire in 21 days, use the Paste Snippet button to create a page containing your info, then copy the resulting URL (address) of the page created.
3. Open a reply to this post, and paste the URL to your troubleshooting information you just copied.
4. Please also post the headers from a returned email using the Mozilla pastebin as instructed above. Two urls for two lots of information.. Message source can be accessed with Ctrl + U it will be up to you to work out where the headers end and the email body starts.

Hi Matt,

Thank you for your reply. There have been some developments since your reply. Now Outlook emails and webmail are being bounced as well as Thunderbird messages which makes me think that it isn't a Thunderbird issue, although, to be honest, I'm going round in circles.

I tried to follow your instructions but https://pastebin.mozilla.org wasn't bringing up a website. Is there a different site to use?

The strange thing is that webmail and Outlook messages were going through until today. The other strange thing is that it is only happening with one client.

Thanks for your help.

https://discourse.mozilla.org/t/pastebin-upcoming-decommission/140759

You could try https://pastebin.com/ enter your paste, set expiration to one week and click the 'create new paste button' - that generates a URL you can share.

Hi there, apologies for the slow response, we have been away on holiday.

The IT person at the company which I am having trouble sending email to suggested that I use redsfift.com to check out what might be happening. When I send an email from Thunderbird I get a different response compared to when I send using Webmail. Screenshots attached.

When I send via Thunderbird I get a DKIM evaluation failed. The reason given is:

   fail
   This is the raw error:

   {
       "error": "bad signature",
       "explanation": "body hash mismatched",
       "source": 0,
       "tag": "bh"
   }

Does that help pin down what may be the issue?

Following on from your previous reply, here is the PasteBin link for the troubleshooting information - https://pastebin.com/MN8nFV1g

And this is the bounced email source pastbin - https://pastebin.com/14JR1q6u

Many thanks!

Hi,

Unfortunately my previous response was auto-deleted when I tried to add the Paste bin links. But, there is other news. The IT support guy at the company who keep bouncing my emails suggested investigating via redsift and I got a different report when sending from Thunderbird as opposed to from Webmail. Webmail has no DKIM issues, but the email sent from Thunderbird has an error. (See attached images). Does that give a clue as to what might be going wrong?

Thanks!