My address book correctly shows c6jdb@btinternet.com, but when I write an e-mail to this person it is sent to c 6jdb@btint!ernet.com & doesn't arrive!

I've tried editing the e-mail address in my list of addresses and also taken it out completely and put it in afresh and it shows correctly as being c6jdb@btinternet.com , but as soon as I enter John on the first line of the address when writing to him the address comes up as c 6jdb@btinternet.com and, because of the space between the c and the 6, he does not receive the e-mail even though I have the message that the e-mail has been sent and confirms that it has been included in my SENT LIST! Sure enough, I can go to that list and the e-mail appears there and confirms it was sent and the time it was sent. What it does not do is tell me where it was sent to because John does not receive it! I hve no problems with any other of my addressees and I have used Thunderbird very successfully - and contentedly - for years, just as I have used Plusnet as my provider.