Just checked that with sending to my hotmail account and it worked just fine here in TB38.1.0.

Who is the mail provider you're using when it fails? I used gmx.

Oh, and RFC 5322 is dated October 2008 and is pretty general, so the error message is remarkably unhelpful. And the date means that it's not an issue of Thunderbird not being up to date with some recent change.

I use 1and1.co.uk as a mail provider.

My first message (bcc'd to a long list of emails, an address book list) bounced all the hotmail ones using 38.1.0

All went through (or more accurately, no reject message from hotmail. I haven't checked that any of them were actually received) after re-installing 31.7.0 I didn't change anything else.

I have seen messages elsewhere about hotmail being picky about all header fields being present and a particular form but nothing that seems to apply in this case.

Ok lets see if we can work this out in the real world.

I just tried to create a Hotmail account, but the best I could do was an outlook.com. So I tested sending to the email address as hotmail. No dice It is an outlook.com address. I would hazard a guess that the issue is either an add-on or some sort of security software at your end. However I think in an effort to establish what is happening you could send me some mails.

So I will give your the address devilsgatedrive both at gmail and hotmail dot com. Can you send the message that was rejected using 38.1(or any message that has been rejected really. Use edit as new from your sent folder) to those two addresses If it arrives at both then the test is a failure. However if it bounces at hotmail and delivers to gmail I will have a header to work with.

It would also be helpful if you could send the same mail with the version that is working for you, 31.0.7?. Once we work out what is the difference in headers, then we stand a chance of moving forward.

The only thing I would expect to change is the the user agent string as it is version specific. But the string from my daily build was accepted so I see no reason any other would not be.

Mine shows as

User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0) Gecko/20100101
Thunderbird/42.0a1

I created these email addresses for the sole purpose of testing, so if you bang away at them more than would usually be acceptable I will not call you a spammer.

Ok I have the what, Not the why.

The what is. 31.0.7 inserts the message header

From: Roland Gifford <roland@ragsystems.co.uk>

38.1 Inserts

From: Roland Gifford (which is not RFC compliant.)

So now I suggest looking to add-ons. most likely those that purport to stabilize the sender as they would be directly manipulating that.

As a general sweep, you could start in safe mode by holding the shift key while starting Thunderbird and thus disable all add-ons just to see if a send works for you. Then start working out which is the culprit.

Thanks for that

I can guess the culprit as being Virtual Identity 0.9.33 from here https://www.absorb.it/virtual-id as that has the purpose of manipulating the from address.

I use different email addresses (for the same domain) for virtually every web site or company where I have an account. I know that Thunderbird has the capability of defining Identities but I have 100s of these so some tool which replies from the address that the message was sent from is ideal. It also knows which from address should be used for each to address when writing new messages. I will have an experiment with settings and will continue to use your test email to see if I am able to fix it.

I haven't been able to find anything other than Virtual Identity which does this.

Just in case someone else has this problem and finds this thread by searching.

Virtual Identity was only partly the culprit. My TB GMail account had a single identity against it with a blank email address. I've no idea why and couldn't fix it without creating an additional identity and deleting the old base one.