The Mishnah in Masechet Sanhedrin (90a) states: “One who utters the ineffable name of Hashem through its letters has no share in the World to Come.” Rabbeinu Chaim Vital writes in the name Rabbeinu Ha’Ari z”l that “uttering Hashem’s ineffable name through its letters” means that it is prohibited to pronounce the ineffable name of Hashem composed of the letters “Yod” and “Heh” and “Vav” and “Heh” at any time. Even during blessings and prayers one may not pronounce the name of Hashem the way it is written; rather, one must pronounce it as the name of Hashem referred to as “Adnut,” which is composed of the letters “Alef,” “Dalet,” “Nun,” and “Yod.” (Certainly, it is permissible to use the “Adnut” name of Hashem only in the context of blessings, prayers, or Hallel, for uttering the name of Hashem in vain constitutes a Torah prohibition.)
This is indeed the prevalent custom among the entire Jewish nation in that although the name appears in Siddurim in its original spelling of “Yod”, “Heh”, “Vav”, and “Heh”, it is pronounced as the “Adnut” name.
Rabbeinu Ha’Ari z”l adds that it is likewise forbidden to speak out the letters of Hashem’s ineffable name (as such: “Yod” and then “Heh” and then “Vav” and then “Heh” consecutively), for this also constitutes the name of Hashem which is prohibited to mention. One who utters it in this way has no share in the World to Come either. Maran Ha’Chida writes that this constitutes a lack of respect toward Hashem, for as we have discussed in the laws of honoring one’s parents, one may not call one’s father by his first name; similarly, one may not mention the name of one’s Father in Heaven. Mentioning Hashem’s name is indeed even more stringent, for one may speak out the letters of his father’s name, for instance, if one’s father’s name is Yaakov, one may speak out “Yod,” “Ayin,” “Kuf,” “Bet”; however, the name of Hashem may not even be mentioned in this manner, as we have explained. It seems from the words of the Ari z”l that the prohibition of uttering Hashem’s name in this way is more severe than actually pronouncing Hashem’s ineffable name as it is written.
Thus, the Poskim write that even while reciting the “Leshem Yichud” texts printed in Siddurim, one must take care to say “Le’Yachada Shem Yod Keh Be’Vav Keh” and not speak out the letter “Heh” (i.e. replacing “Heh” with “Keh”), for the alternative would constitute the prohibition of speaking out the letters of Hashem’s ineffable name.
The Poskim disagree on whether it is prohibited to speak out the letters of Hashem’s “Adnut” name, meaning to say “Alef,” “Dalet,” “Nun,” “Yod” or is this only prohibited when one speaks out the letters of the name of Hashem as it is written. Maran Rabbeinu Ovadia Yosef zt”l rules that the letter of the law dictates that one may be lenient in this matter in accordance with the view of the Radbaz (although the Tosafot [Sukkah 5a] write that people customarily abstain from speaking out the letters of the “Adnut” name of Hashem as well; if one can be careful regarding this matter in accordance with this opinion, one is especially praiseworthy).
Maran zt”l adds that even regarding the ineffable name of Hashem, if one does not intend to utter the name of Hashem, there is room for leniency. It is therefore permissible to recite “Abaye Hava Mesader Seder Ha’Ma’aracha” in the Shacharit prayer without pausing between the words “Abaye” and “Hava” although this may sound like he is uttering Hashem’s ineffable name; it is nevertheless preferable to make a short pause between the words “Abaye” and “Hava.”