Warning letters simply document the incident and are valid for nine months from the date of discipline. (I don't believe this language is differen't in any other agreement but check yours). They 'drop off' and a new one must be issued for a recurring offense.
While the Warning Letter will specify that you can be terminated for any future infractions, that is rarely the case. Usually progressive discipline is followed and this has been upheld by the courts as being ingrained.
In my local we actually have a signed company agreement that specifies progressive discipline for all infractions regarding attendance as documented talk with, WL, 1day suspension, 3 day susp, 5 day susp, and then Termination. For part-timers they add two extra "documented talk withs" in order to 'fix the problem' and reduce turnover. This has been very successful and I used the model to reduce warning letters to package drivers.
We came up with a form, called an "EQUIP" Form, that would serve as a 'documented talk with' when addressing service failures with package drivers. This reduces what I call "training by discipline". Management, the employee, and the steward sit down and discuss the what caused the service failure, what methods weren't followed, and then ask the employee to suggest ways to address the problem with the entire center. This completely changes the tone of the meeting. It can be difficult to get your management team to sign on to the program, but it works.
Management likes it because it gives them a form to show that they have spoken with and re-trained the employee with a steward present. This form can later be shown when further discipline is warranted.
The Steward likes it because 1. it isn't really discipline and adds one step to the progressive discipline process and 2. it allows the steward to get the employee to 'shape up' and hopefully avoid further discipline. Also, it engages the employee in the process, allows the steward and employee to challenge procedures that may be leading to service failures, and sometimes discovers personal problems such as family issues that may be contributing to the employees work-related problems.
Sometimes I would even get to avoid the next step of a warning letter by getting management to allow me to have a 'heart to heart' talk with the employee after an EQUIP form had already been filled out.
Back to your original question, when Warning Letters are sent certified mail (as is required by the contract) it doesn't matter if you receive it or not, simply that they send it. I told all my drivers to accept it and give me a copy, as the other one goes to the Union Hall and can be difficult for me to retrieve. I would check the letter for any errors as far as date, offense, and people present as many times they do contain errors. Some lacky in H.R. writes and signs these, your actual management doesn't even see them. Any errors will make the warning letter invalid which can aid avoiding future discipline.
90% of warning letters meetings I sat in on were due to methods failures or attendance. Two easy things to fix. I never understand why someone with a $70,000+ a year job can't show up for work!