Not in talking terms with spouse and they refused to provide SSN, I am filing MFS. What documents do I need to send when mailing in the tax papers.

[Happymonkey1234]

I understand that when MFS we do not need spouse SSN and need to mail in the papers, however I wanted to know what additional documents do I need to send along and what should be the content of my cover letter explaining why I could not provide my spouse SSN?

[MinhT1]

If you do not have your spouse's SSN when filing as Married filing Separately, you must file by mail and include a note explaining why the spouse's SSN is missing. You can describe your efforts in obtaining the SSN (not in talking terms).

[Happymonkey1234]

Thanks for replying, are personal reasons enough to explain the missing SSN..? 
Do I need to send other documents to help identify him such as passport copy, etc?
Do these kind of situations pose higher chance of rejection? Or the only downside is delay?

[MinhT1]

You can just give your spouse's name in the note. Your tax return will not be rejected because of the lack of your spouse's SSN. It just cannot be e-filed.

This IRS document says, on page 3-5:

A spouse who is Married Filing Separately is not required to provide the Social Security card for the other spouse, although the return cannot be e-filed without the spouse’s Social Security number.