Most recent landlord requested rent be sent to po box. I sent on 2nd or 3rd... she did not receive rent within two weeks of due date and slaps 3 day notice to pay or or quit on my door... rent addressed to po box was returned by mail on the 18th not deliverable as addressed unable to forward... this is the second time this has happened I am trying to do the right thing but she makes up rules as she goes and harasses me even if there is a chance it is a mail box issue and not me... she lives behind me and thinks banging on my door every day is going to make it better... I am very stressed and cannot afford to move... on a month to month but just want her to abide by the lease and she won't... what can I do.. I live in San mateo county California...

First, A landlord can require rent payments be sent to a PO box. Second, If a rent payment is late by even one day, they can give you a pay or quit notice. This still has to go to a hearing which, hopefully, will not occur until the mail is returned and you can show the court that you did attempt to pay on time.

I don't see anything in your post that indicates she is not abiding by the lease. But, as a month to month tenant she can terminate your tenancy with 30 days notice (60 if you have lived there more than 2 years).

If she lives behind you, why can't you just hand deliver the rent check?