What is the difference between "to be" and "must/need/should be"?

It is a very General question, but here is a starting point, in very general terms:

Use "to be" when that is what the future holds, according to the current situation.

For example:

• OK: "My house has been marked DANGEROUS by the authorities, which means that it is to be demolished [shortly, by the authorities] ...."
• Not OK: "My house has been marked DANGEROUS by the authorities, which means that it should be demolished [by me, whenever I have the time] ...."

Use "should be" when that is what you want or feel or think.

For example:

• OK: "Your house looks weak; it should be renovated [at your expense & convenience] ..."
• Not OK: "Your house looks weak; it is to be renovated [right now, by order!] ..."

In your example of email address deletion, either can be used without much change in meaning. Deletion is what the future holds, when you click the Delete button, and this is exactly what you want to happen.

In fact, the modals "must" and "should" can be used in the definition of this verb form ; it is considered to be an auxiliary.

(OALD) 5 be to do something used to say what must or should be done
♦ I am to call them once I reach the airport.
♦ You are to report this to the police.
♦ What is to be done about this problem?

TOPICS: Permission and obligation

There is however a second meaning of this auxiliary.

(OALD 6 be to do something used to say what is arranged to happen
♦ They are to be married in June.

It follows that in the case of this indication about an email address, there may be a difference or there may be relatively little. It depend on the procedure which precedes. If, in order to suppress the address, certain requirements must first be complied with, then this prerequisite can be seen as an arrangement between user and sevice provider; in this case the meaning is not that the user sees the supppression as a necessity (although the necessity is a reality) but it is instead "the suppression will be carried out as compliance with the preliminary steps makes necessary".