Hello @Thomas_Girard ,
After some discussion with Braintoss folks it looks like it should be working now. At least the e-mail address like post+1d2e8ab25ba4b6651396eff5fe494d5e+614108@checkvist.com is fine with them.
What is the length of the e-mail you have in your “Email-to” address? If it is too large, you can try re-generating OpenAPI key in Checkvist on the profile page (
Note: this will invalidate all your previously generated email-to addresses in Checkvist and all the custom integrations if you used any).
Some time ago I increased the length of the OpenAPI key, and then decreased it back, and this probably affected the Braintoss integration in the process 
Hope this helps,
KIR