How to stop Win 10 with Outlook sending attachments as Winmail.dat

I have just joined this forum. Am an XP user but have been asked by a friend to help set up e-mail on a Win 10 laptop running MS Outlook.
Mostly it works fine but sometimes encodes attachments as Winmail.dat files which recipients can not open or use.
This problem is widely reported on internet.
I have tried every solution I have read, without success. These include ensuring that Outlook sends messages as HTML or Plain Text (NOT as Rich Text) and applying a change to the registry to inhibit poduction of Winmail.dat files. Yet still the problem persists.
Sending to another Outlook address is OK. Sending to a GMail address works but sending to a BTInternet address never works in that all attachments (doc / docx / jpg etc.) get sent as Winmail.dat which I can not open. Moreover I use MS Outlook Express to access my e-mail accounts and this program fails even to recognise Winmail.dat!
It seems as if certain ISPs interact with Outlook and cause it to send Winmail.dat attachments.

Is anyone aware please of a hard and fast solution which guarantees that OUTLOOK will NEVER generate Winmail.dat attachments?

CMOORE100

Ken,
Many thanks for your very rapid and helpful reply. I am provisionally of the view that yur solution has solved my problem. However I would like to evaluate it a bit more and ensure that there are no bugs (none so far). I will then post a full reply describing all which I have done to overcome what seems to be a well known problem with WIN 10 / Outlook which MS appear to be aware of but have failed to address either by knowledge paper or, preferably, software update.
Will post further in2 or 3 days, but am optimistic thanks to your input Ken.
Very many thaks.

WIN 10 (64 bit) with OUTLOOK 2016 send E-Mails which some recipients are unable to open or read.


The problem most commonly encountered, to judge from many internet pleas for help, is that normal e-mail messages are sent and received satisfactorily but attachments – whatever their file extensions – are sometimes apparently not sent or not openable. There are also reports that some e-mail main texts are apparently not sent or not openable. This note is based specifically on WIN 10(64) plus OUTLOOK 2016, but my extensive reading for a solution has revealed that other WIN / OUTLOOK issue combinations also give rise to the same or similar problem. It may therefore well be possible that the following actions will also be of assistance with these.
The root problem is that OUTLOOK sometimes encodes messages and/or attachments as WINMAIL.dat files and sends these. Many non-MS mail programs can not recognise and/or open these. WINMAIL.dat is a format used in times past for sending Rich Text Format (RTF) messages. I am uncertain why MS persists with this heritage I have found in limited testing that some ISPs seem to receive OUTLOOK attachments OK (because these are sent as their original files with “normal” extensions) whereas another ISP appears to prompt their conversion to WINMAIL.dat and then can not open them. I have, by the way, eliminated any possibility that the problem stems from my receiving computer. My accounts are of the “web based” type and I can open my E-mail boxes by web access direct to the ISP.
My testing (not comprehensive) has shown that when OUTLOOK is used to send attachments to a Gmail.com account they are received OK but when the exact same attachments are sent to a BTInternet.com they are received as WINMAIL.dat encoded files. It is thus doubly unsatisfactory in that one can not rely on the receipt of attachments sometimes, but not always.
Here are the 3 steps I took :- 1.. Modify the registry to inhibit generation of Winmail.dat files. To do this a.. Open program REGEDIT (Windows key plus R) b.. Go to Computer/ HKEY_CURRENT_USER/ SOFTWARE/Microsoft/Office/16.0/Outlook/Preferences c.. Create a new DWORD(32bit) called Disable TNEF
d.. Enter a value of 1 ie.0x00000001 (1) e.. Exit Regedit 2.. Set up OUTLOOK so as NOT to send messages in RTF format. a.. Open OUTLOOK b.. At top left SLCk the blue FILE tab. File page opens.
c.. In list at LHS SLCk OPTIONS. d.. From OPTIONS find and SLCk MAIL. e.. In MAIL option scroll down to COMPOSE MESSAGES f.. Find CHANGE EDITING SETTINGS g.. Select PLAIN TEXT format option in box. (HTML option may also be OK but RICH TEXT FORMAT – RTF – must NOT be selected). h.. Close the MAIL options page(s).
3.. Set up OUTLOOK not to auto-complete addressees and empty the auto complete cache. a.. Open OUTLOOK b.. At top left SLCk the blue FILE tab. File page opens. c.. In list at LHS SLCk OPTIONS. d.. From OPTIONS find and SLCk MAIL. e.. In MAIL option scroll down to COMPOSE MESSAGES
f.. Select SEND MESSAGES section. g. UNCHECK box labelled “Use auto complete list to suggest names when typing To/cc/bcc lines”. h.. Find and SLCk the “Empty auto complete list” i.. SLCk YES in response to “ARE YOU SURE” pop-up box j.. There is no apparent response to step i. k.. Close OUTLOOK mail option page(s). l.. For good measure re-start the PC.

Notes : i.. At step 3e with OUTLOOK 2013 scroll to SENDING MESSAGES section. ii.. In OUTLOOK 2003 and 2007 for procedure section 2 : iia.. Access OPTIONS from the tools menu iib.. Click MAIL FORMAT tab iic.. Click INTERNET FORMAT iid. Under Outlook Rich Text Options click either Convert to HTML format or Convert to Plain Text format. Get rid of any reference to RTF.
iii. This note is written based on Win 10(64bit) plus Outlook 2016. Other OS/Outlook versions may vary in function and detail.
iv.. Some versions of OUTLOOK provide for the message sending format to be varied between addressees in the address book. To check for this each address book entry needs to be examined from its PROPERTIES values. These should be opened and set to HTML or PLAIN TEXT and never RTF (or TNEF). v.. With my problem the solution was achieved after step 3. It appears that the auto-complete cache “remembers” previous settings and hence it needs to be deleted.
vi.. Step 3 was the vital action and was kindly contributed by Caledon Ken at TEN FORUMS I will mark the thread as resolved after further evaluation, but am highly optimistic. .