I am sending html email with content-type set as "multipart/alternative". The mail is displayed fine on other email clients like gmail, but in Zoho, it simply shows no content.

When comparing, I find that Zoho is adding the boundary incorrectly in the headers.

Mail to gmail:  https://pastebin.com/xYtRWc3t

Mail to zoho:  https://pastebin.com/fpvHiWXf

The only thing that I see while comparing the two emails is that Gmail is not making any changes in the mail boundary (check lines 41-45 in the 1st link above), whereas Zoho is modifying the boundary string which it should not (I am referring to line no 25 in the 2nd link). This is completely breaking the email format, and results in a blank email in Zoho.

Any advise to overcome this will be helpful.

Regards,

Vimal