You're SOW (Statement of Work) should clearly define what is to be delivered, who is responsible for what, what is in scope vs. out of scope, and the schedule for execution/delivery. 

The property must be vacated by xx/xx/xxxxSignedXxxxxxPlace somewhere on the letter the method of delivery be it in person, taped to the instof the door or certified mail

Another avenue is to ask the local postal carrier (if there is still home delivery in the neighbourhood) or to make friends with the property management companies who look after the properties for the lenders.

Network with service people, i.e. letter carrier, pool service person, lawn service person, newspaper delivery person, carpet cleaning people, plumbers, etc. 23.

Send the letter in a priority mail envelope and delivery confirmation to see thAt they received it.

You more than likely used more of a standard promissory note which attached to the DOT/Mortgage or you paid extra in legal fees to create one suitable to your state standards.The fears that coincide with notes and security instruments have all been handled and much of what exists today is just street level myth from incorrect interpretation: ‘transfer of the note carries with it the security, without any formal assignment or delivery, or even mention of the latter.’

Your cleaner should act as your primary "boots on the ground" and should be able to do things like pick up supplies when needed, set out amenities and accept packages/deliveries for your property.

If you can be a reliable go-to type contractor who need minimal babysitting and occasionally anticipates the investor's needs ("hey the subfloor is all rotted out in the bathroom, do you want me to refloor over it or I can replace it real quick") or goes above the call ("I'll stick around a 1/2 hour to let the cabinet delivery guys in so you don't have to drive cross town in rush hour just to unlock the door"), you don't need to be rock bottom on price.

Go to  https://www.biggerpockets.com/settings/notifications to change the frequency of your alerts.You may want to add the keyword alerts back in, and if you're getting too many, you can change the frequency of their delivery or make them more specific.

How can I meet the mail delivery workers in the towns (out of state) where I am marketing to?