Is sending an email adequate notice of rent increase?

No, it has to be in writing, with an ink signature. I have even seen judges feel the signatures to make sure they have the ink copy.

No. It needs to be in writing and the increase can't start for 30 days

No it is not adequate notice and will not stand up in court.

In person or by mail are the only methods listed for California. Also, if all tenants have been there more than a year then 60 day notice is required; not just 30.

"A landlord's notice of rent increase must be in writing. The landlord may deliver a copy of the notice to you personally. In this case, the rent increase takes effect in 30 or 60 days from the date the notice is delivered.

Alternately, the landlord may mail the notice to you, with proper postage and addressed to you at the rental unit. If the landlord mails the notice, he or she must give you an additional five days' notice. That means the landlord would have to give you 35 days' notice from the date of mailing if the rent increase is 10 percent or less. If it is more than 10 percent, 65 days' notice is required."

No, and neither is a text message. When dealing in real estate matters, the courts rarely recognize anything other than old fashioned 'black on white', delivered via conventional means (personal delivery or mail)

Opinion: No.

People can change their email addresses or simply not check.

It would also be extremely unprofessional to do this.

Do this by mail, in person, or note on their door.

you are able to in basic terms deliver the interest by technique of familiar mail. besides the indisputable fact that, in case you want data that the tenant surely gained the interest, deliver it qualified, go back receipt. qualified is more cost-effective than registered yet you fairly want the go back receipt which proves the tenant gained the mail.