It is clear from reading the second stanza that the speaker's grandmother eagerly anticipates the arrival of the mail, hoping to hear from her son in America. She knows that the mail doesn't arrive very often and that there's seldom a letter when it does. But she sits there waiting patiently, all the same, keeping herself occupied by baking bread.
Whenever a letter arrives, she's so excited that she announces it as a miracle. She listens to its being read over and over again in the dim light of evening. This indicates that the speaker's grandmother is illiterate. And yet, despite her illiteracy, the written word has the ability to excite her and to bring happiness into her life.
What matters to her most of all is not her ability to read but the contents of the rare letters delivered to her home. They make her happy, adding to the sense of joy she feels on a daily basis due to her firm belief in Allah, who she says is everywhere, even in death. He is her first thought every time she wakes up, and his message is even more significant than that which she receives by mail.