If lack of mailing is your only defense it will boil down to whether the landlord admits improper service. If so, you should win this round. If not and the landlord presents evidence of posting and mailing it will be determined by who the judge believes -- the person who testifies as to service of the 3-day notice, or you.
If you wish to remain in the rental unit, can afford to bring your back rent current, and reimburse your landlord for the costs of the UD, it is not too late to negotiate a deal.
Defective service will simply require the landlord to begin anew. Meanwhile the amount of unpaid rent and costs just increases and makes any amicable resolution more difficult.
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You don't indicate how many days ago you were purportedly served by posting. In some instances,the landlord already obtained a court order to permit you to be served in this manner.
Your remedy would be to file a Motion to Quash, not an Answer.
Generally speaking, your chances of winning an unlawful detainer as a tenant are very small if you do not have an attorney and if the reason for the eviction is due to non-payment of rent where habitability issues are not present.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.