Word can open and save .txt files, so that is not the issue. The issue is that .txt files absolutely cannot contain the mergefields necessary for a mailmerge. Given that stricture, how do you propose any application would be able to complete what you're calling a mailmerge?

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I am aware that Word is the best tool for mail merge, but it is not the only one. OpenOffice can do it too. Today, I believe that the term 'mail merge' is more related to MailChimp and Gmail than Word. Unfortunately, none of them can solve my problem. Anyway, I found this free web app called DocBlender that supports .TXT templates, but doesn't have API support (although they mention it on their website).

You posted on MSOfficeForums.com - which is quite obviously geared to supporting users of Microsoft Office products. You can hardly expect replies here to address what you may or may not be able to do with a non-Microsoft product. Moreover, it was you who specified:
notwithstanding that:
which quite obviously mean they're not mergefields. Hence your placeholders would have to be replaced with mergefields before a mailmerge could be implemented.

If you don't want a Microsoft Office product - which essentially means Word or Publisher - I suggest you ask elsewhere.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]