Thunderbird uses the entries in identities to find which SMTP server to use to send out mail. In the absence of an identified identity, it uses the default for the current account.

While there is an option to allow a one off modification of identity in the compose window it has never been the intention to have the to: address be just sent from a random SMTP server. Most mail providers actually actively disable such, with hotmail/outlook simply ignoring what you set and reverting it to the defaults for the sending account on their server and gmail refusing to play unless you jump through hoops to confirm ownership of the sending address.

Basically the days of spoofing a from address are about over.

Matt,

Thank for your help. However, the more I more I read it and think about it, the less I really understand it. I will cover 'spoofing', something which I do regularly, later.

All the emails I send out from me come from a domain I own. Most are of the format *@kemputa.tld where * is anything of my choice and 'tld' is .uk, .com etc. each different 'tld' used creates a domain that is registered and paid for. (I also own a few domains that do not contain the word 'kemputa'.) As far as I have noticed, when Thunderbird tries to change who the email is from, the sender address I want and the sender address it picks are normally identical from the '@' onwards. (I.e. it has not elected to change the domain.) I cannot understand how this causes confusion with a SMTP server. For both in and out, the two servers involved handle the entire domain, and will inherently merely pass through anything belonging to that domain.

When I 'reply' to an email, without any messing about, Thunderbird creates a skeleton into which I can merely type my actual reply body. Then what is displayed is 100% correct and what I want. All addresses, the chain, everything is perfect. When I press 'send' I get the dreaded yellow message. Rephrase:- 'The sender address will bodged to make the message appear to be from someone who has nothing to do with the conversation'. If this were to happen and the recipient presses 'reply', my own system software may route the reply to the person Thunderbird chose, as he has now become a 'to' address. I have never sussed exactly what happens, but it has caused embarrassment, and I am not personally aware the reply even exists. Furthermore, there is the danger the wrong address has been noted as an address to use for me.

Going back to the point where I first got the dreaded yellow message, and the 'from' address is now fouled up and not the correct one that was there before I pressed 'send'. The yellow message merely has a cross after it. I have never recently risked clicking it, I do not know if the message would not be sent or sent from the wrong address. My memory suggests the later. I cancel the send. The from address displayed is now messed up. When I remember I have had the forethought to copy it to my clip board before I pressed send. I now have to press 'change send address', delete the fouled up send address, and paste it back to exactly what it was. This time Thunderbird will let me use it, and for that message all is now perfect, never ever creating any further hoops or issues with the posting. To me the whole episode seems quite illogical and stupid. I need an option or work around on the yellow message to say 'no don't change the send address or anything else, just get on and post it exactly as it is'. In all this, I never feel close to creating any issue with any server, or have 'spoofed' anything.

What I call 'spoofing' is when I do send from an address that I do not own in any way. My family provide a service to people technically out of their depth. The main person who goes around sorting out individual computer calamities is my eldest son. However, people cannot always wait for him to be available, they want their latest message posted. If their problem did not prevent it, they would give me exactly what they want on a memory stick. Otherwise they would type on one of my computers & probably loose any chain. Sometimes they want a picture attached from a server of mine. I essentially have the job of being given the equivalent of a paper letter to be put in an envelope, addressed and 'fromed' as requested, and posted. The 'from' is theirs, and any reply is to be to them, I don't want it. Oddly, Thunderbird works extremely well for this. I create 'new message', and change the 'from' and 'to' address to be the ones provided for the purpose. Why should the recipient care or even know who actually posted it? (If they ask me, they should be prepared to be bored to tears with the detail of my reply.) It is rare for me to have an SMTP server objection, but if I do, Thunderbird easily lets me temporarily use one of the SMTP servers that allow sender to be anything. (Normally use of these is free for a limited number of postings in a given time period.) For this, Thunderbird is user friendly. What do you mean the days of spoofing a from address are about over? I hope this is not about to change, I do this a lot.