Tenant says she didn't receive 60 day rent increase California

[Susan O.]

I have a tenant claiming she did not receive the rent increase. For the past couple years we have sent two.

I mailed it through regular mail.  It's for a rental in California.  Non-rent controlled.

The rent is automatically paid and we increased her rent on her auto payments and she is now claiming that all the previous payments weren't valid.

Isn't the letter in regular mail sufficient?  She's now claiming I need to reimburse her this past rent and she claims it invalidates the current increase.