I apologize if I misinterpreted your original question. You are quite right that if you do not actually have a separate maintenance decree issued by the court, that it is not the same as simply having a separate maintenance agreement drawn up by an attorney.
In your situation, you would not qualify to file as a "single" taxpayer. However, you may qualify to file as "head of household" if you meet certain criteria. The filing status of head of household actually offers a bigger tax advantage than filing as single.
This can apply to you even if you are not divorced or legally separated. If you qualify to file as head of household, instead of as married filing separately, your tax may be lower, you may be able to claim the earned income credit and certain other credits, and your standard deduction will be higher. The head of household filing status allows you to choose the standard deduction even if your spouse chooses to itemize deductions.
Here are the criteria you must meet to file as head of household:
You file a separate return.
You paid more than half the cost of keeping up your home for the tax year.
Your spouse did not live in your home during the last 6 months of the tax year. Your spouse is considered to live in your home even if he or she is temporarily absent due to special circumstances.
Your home was the main home of your child, stepchild, or foster child for more than half the year.
You must be able to claim an exemption for the child. However, you meet this test if you cannot claim the exemption only because the noncustodial parent can claim the child.
If you meet all of the above criteria, then you can file as head of household, even though you are not "legally" separated by a decree from the court.
I am giving you two links below. One is to IRS Publication 501 which explains the rules for filing status. Refer in particular to the section for "Married Filing Separately" and then also to the section for "Head of Household - subsection "Considered Unmarried".
The other link is to the actual IRC Code 7703 which is the actual law which governs this choice in filing status.
I hope this helps clarify things for you, but if not let me know.