My "guess" is you are running into a particularly strict spam filter on the persons account. It could be a strict as to not deliver to the inbox unless your address is in the recipients address book.

There are many degrees between everything gets through to only those in the address book get through. Historically the most common stumbling block are email signatures, especially if you can send in web mail and get it delivered but not in Thunderbird. The issues with signatures are usually related to images for social media web sites as they have previously been used by spammers. But just the existence of a signature can be enough.

Then we have folks that are sending using their ISP server and an email address for another domain. This is a key indicator of SPAM. For instance sending for George@gmail.com using a outgoing mail server for Comcast, instead of the Gmail one.

Matt, Thank you. I don't use signatures on the accounts I used to send this from PC or phone. I checked with my contact and he has my email in his contact list on both outlook and Thunderbird. Whilst my contact uses webmail, I sent it on desktop version of MS Outlook (as part of MS365). The SMTP settings are: Google Mail, Server name: smtp.gmail.com Port 465 Connection security SSL/TLS Authentication method: OAuth2 Username: my name@gmail.com

I appreciate this is very odd and that the answer may be very obvious. Not obvious to me :-( Thank you

I doubt it is obvious to anyone. One of the big downsides of email is it actually has no guarantee of delivery. So once your provider accepts the email, tracking what and why can be most difficult to nigh on impossible.

Check in your Thunderbird account settings that there is no reply to address specified for the account. I have seen that cause issues, despite it's existence being perfectly acceptable in the standards. Unfortunately these huge players also play hard and fast with the rules. Their size lets them be the elephant in the room.

Send me an email to mattau at gmx dot com preferably close or the same as what is not being delivered, certainly from the same account. Perhaps I can see a clue in the message source.

Hi Matt Thank you. Correct, the "reply to address" box is empty. When I sent an email to mattau@gmx.com I get an error message that "Address not found Your message wasn't delivered to mattau@gmx.com because the address couldn't be found, or is unable to receive mail." Have I misunderstood the email address?

Matt, I just noticed the time stamp. I forget time zones :-(. Sorry. John

Matt, When I tried to send a msg to the recipient using gmail in a browser, I noticed the attached information when clicking on the red lock next to their name in the msg. Is this relevant? It does not appear on the other contacts I have that have @outlook.com addresses.