A. Which letters are permitted for one to read?
Reading a letter from a friend: A [common] letter [of a friend or relative] inquiring ones wellbeing is forbidden to read [verbally], and even to read in one’s mind without verbalization [is forbidden] due to a decree [that one may come to read] layman documents.
Letters regarding finding objects: (It goes without saying regarding letters sent to inquire about a certain matter, even if it is a matter which is permitted to do on Shabbos, [that it is forbidden to be read] as they are to be decreed against due to [one may come to read about] matters which are forbidden).
Other Opinions: There are those opinions which permit reading a letter which is sent to him of which its content is unknown, as perhaps it contains matters which pertain to his body, and it is thus not similar to layman documents which have no pertinence to one’s bodily [health and security], but rather pertain to ones money.
The Final Ruling: One may rely on their words to be lenient on a Rabbinical matter. Nevertheless one is not to read it verbally but rather in his mind, without verbalization, as there is an opinion which is lenient regarding reading [in one’s mind] without verbalizing even writings which are forbidden to read according to all.
B. Are forbidden letters Muktzah?
Letters are not Muktzah: It is permitted to move the letter on Shabbos and it is not forbidden due to Muktzah even if it is not fit on Shabbos for its main benefit, which is to be read, such as when one knows it does not contain matters pertaining to ones bodily [health and security], in which case it is forbidden to even read it in one’s mind.
The reason for why it is not Muktzah is because: Nevertheless it is fit for another benefit, such as to cap a bottle with and matters of the like.
If the letter arrived from outside the city boundaries: Even if the letter was brought from outside the Shabbos parameter on Shabbos it is not Muktzah, as is written in chapter 515 [Halacha 14], and even the person for whom the letter was brought for is permitted to benefit from it even from its main benefit, which is to read it if he is unaware of its content.
The reason it is permitted even when arrived from outside city limits: It is not similar to other items brought from outside the city limits which is forbidden for the person to whom it was brought for to benefit from until after Shabbos, as the reason [there is] due to a decree that one may tell a gentile to bring him an item from outside the city limits, however by a letter this is not relevant [as one does not know when a letter is arriving]. Furthermore, this letter was not brought on his behalf at all being that the sender sent it for himself and not on behalf of the one it is being sent to. (Furthermore, even if it was sent on his behalf, nevertheless the gentile which is bringing the letter from outside the city limits intends on doing the task of the sender and not on behalf of the person it is being sent to. It is not similar to a Jew which sent fruits to his friend before Shabbos through a gentile and he was delayed and brought them on Shabbos, in which they are forbidden for the person to whom it was sent to, as written in chapter 515 [Halacha 22], as since the gentile knows that this Jew which he is bringing them to will eat on Shabbos these fruits and therefore he is bringing them to him, if so he is then intending in his bringing them from outside the city limits on behalf of the bodily benefit of the receiver, and for this reason even if the gentile intends on doing his task for the sake of himself, in order to receive payment, it is forbidden as is written in chapter 515 [Halacha 21]. This is in contrast to a letter in which the body of the recipient does not benefit from the body of the letter, and the gentile only intent in bringing the letter from outside the city limits is to fulfill the will of the Jewish sender, or for his own benefit to receive payment, and not for the sake of whom it is being sent for.)
C. Not to take the mail from the postman’s hand:
It is accustomed not to accept a letter from the hands of a gentile which brought it on Shabbos, but rather the gentile is told to place it on the ground or on the table, as we suspect that perhaps prior to the gentile standing to rest [upon arriving] the Jew will take the letter from his hands, and it is thus found that the Jew has done the “Hanacha” [placing down part of the carrying], which is Rabbinically forbidden even in these times that there are those which say that we do not have a public domain, as is written in chapter 325 [Halacha 2, 16]
Letters [which have had their envelope opened from before Shabbos] of which their content are unknown, and it is thus possible that in them are matters pertaining to one’s health and security, are permitted to be read in one’s mind without verbalization. If however one knows that it does not contain content relevant to one’s health and security, then it is forbidden to read their content even in one’s mind without verbalization. Nevertheless, the letter is not Muktzah, as it is still fit to be used as a bottle stopper. Furthermore, even if the letter arrived on Shabbos from out of city limits it is not Muktzah. 
Taking mail from the mailman: One is not to take mail from the mailman, but is rather to have him put it down, in order so one not come to take any part in the gentile’s act of carrying.
May one open an envelope to remove its letter?
It is permitted according to all to tell a gentile that he cannot read the letter until it is opened and have the gentile understand that he wants him to open it.
 So rules Rashi Shabbos 116b
 Tosafos Shabbos 116b
 Meaning his health, or security. [See Tosafos there]
 There it is explained that matters brought from outside the Techum on Shabbos are only forbidden for that person to who it was brought to and not to others. Thus it cannot become Muktzah, as a matter which is not Muktzah for others is not Muktzah for any. [See 308/89]
 See Q&A below
 Piskeiy Teshuvos 340/29
 So rules M”B 340/41 in name of different Poskim. The Peri Chadash holds that it contains a Biblical prohibition while the Chacham Tzevi maintains that it contains a Rabbinical prohibition.
 Chazon Ish, and so leans to rule Rav SZ”A in SSH”K 28 footnote 15, Shut Even Yisrael 16
 So rules M”B ibid