diff --git a/.env.example b/.env.example
new file mode 100644
index 0000000..583c0ba
--- /dev/null
+++ b/.env.example
@@ -0,0 +1,2 @@
+GMAIL_ADDRESS=
+GMAIL_TOKEN=
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..efabc00
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,29 @@
+# Secrets: personal use only, never committed
+.env
+
+# Python
+__pycache__/
+*.py[cod]
+*.egg-info/
+build/
+dist/
+
+# Virtual environments
+.venv/
+venv/
+
+# Test and tool caches
+.pytest_cache/
+.ruff_cache/
+.mypy_cache/
+.coverage
+htmlcov/
+
+# Generated documentation
+docs/doxygen/
+
+# Editors and OS
+.idea/
+.vscode/
+.DS_Store
+Thumbs.db
diff --git a/Doxyfile b/Doxyfile
new file mode 100644
index 0000000..54b5519
--- /dev/null
+++ b/Doxyfile
@@ -0,0 +1,23 @@
+# Doxygen configuration for the birthday wisher (Python).
+# Run from the repository root:  doxygen Doxyfile
+# Output goes to docs/doxygen/ (git-ignored).
+
+PROJECT_NAME           = "Birthday Wisher"
+PROJECT_BRIEF          = "Sends a random birthday letter by e-mail (Udemy 100 Days of Code, day 32)"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src README.md
+FILE_PATTERNS          = *.py *.md
+RECURSIVE              = YES
+USE_MDFILE_AS_MAINPAGE = README.md
+
+OPTIMIZE_OUTPUT_JAVA   = YES
+EXTRACT_ALL            = YES
+EXTRACT_PRIVATE        = NO
+JAVADOC_AUTOBRIEF      = YES
+AUTOLINK_SUPPORT       = YES
+SOURCE_BROWSER         = YES
+
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES
+WARN_IF_UNDOCUMENTED   = YES
diff --git a/birthdays.csv b/birthdays.csv
new file mode 100644
index 0000000..0f98a48
--- /dev/null
+++ b/birthdays.csv
@@ -0,0 +1,5 @@
+name,email,year,month,day
+Alice Example,alice@example.com,1990,3,14
+Bob Example,bob@example.com,1985,10,17
+Carol Example,carol@example.com,2001,10,17
+Dave Example,dave@example.com,1978,12,24
diff --git a/letter_templates/letter_1.txt b/letter_templates/letter_1.txt
new file mode 100644
index 0000000..4f763c4
--- /dev/null
+++ b/letter_templates/letter_1.txt
@@ -0,0 +1,7 @@
+Dear [NAME],
+
+Happy birthday!
+
+All the best for the year!
+
+Angela
\ No newline at end of file
diff --git a/letter_templates/letter_2.txt b/letter_templates/letter_2.txt
new file mode 100644
index 0000000..415913c
--- /dev/null
+++ b/letter_templates/letter_2.txt
@@ -0,0 +1,7 @@
+Hey [NAME],
+
+Happy birthday! Have a wonderful time today and eat lots of cake!
+
+Lots of love,
+
+Angela
\ No newline at end of file
diff --git a/letter_templates/letter_3.txt b/letter_templates/letter_3.txt
new file mode 100644
index 0000000..5ed537e
--- /dev/null
+++ b/letter_templates/letter_3.txt
@@ -0,0 +1,7 @@
+Dear [NAME],
+
+It's your birthday! Have a great day!
+
+All my love,
+
+Angela
\ No newline at end of file
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..d9561b0
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,22 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "birthday-wisher"
+version = "0.1.0"
+description = "Sends a random birthday letter by e-mail to everyone whose birthday is today (Udemy 100 Days of Code, day 32)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = { file = "LICENSE" }
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+pythonpath = ["src"]
+testpaths = ["tests"]
diff --git a/src/birthday_wisher/__init__.py b/src/birthday_wisher/__init__.py
new file mode 100644
index 0000000..985749c
--- /dev/null
+++ b/src/birthday_wisher/__init__.py
@@ -0,0 +1,4 @@
+"""!
+@file __init__.py
+@brief Birthday wisher: sends a random birthday letter to everyone whose birthday is today.
+"""
diff --git a/src/birthday_wisher/constants.py b/src/birthday_wisher/constants.py
new file mode 100644
index 0000000..bb9bd9b
--- /dev/null
+++ b/src/birthday_wisher/constants.py
@@ -0,0 +1,37 @@
+"""!
+@file constants.py
+@brief Constants of the birthday wisher: file names, placeholder, SMTP settings.
+"""
+
+## Name of the CSV file with the people and their birth dates.
+BIRTHDAYS_FILE: str = "birthdays.csv"
+
+## Folder with the letter templates.
+LETTER_TEMPLATES_DIR: str = "letter_templates"
+
+## File name pattern of a letter template (the number is 1 to LETTER_COUNT).
+LETTER_FILE_PATTERN: str = "letter_{number}.txt"
+
+## Number of letter templates the program chooses from.
+LETTER_COUNT: int = 3
+
+## Placeholder in a letter that is replaced by the person's name.
+NAME_PLACEHOLDER: str = "[NAME]"
+
+## Columns of the CSV file.
+CSV_COLUMNS: tuple[str, ...] = ("name", "email", "year", "month", "day")
+
+## SMTP server of Gmail.
+SMTP_HOST: str = "smtp.gmail.com"
+
+## SMTP port that expects STARTTLS.
+SMTP_PORT: int = 587
+
+## Subject of the e-mail.
+EMAIL_SUBJECT: str = "Happy Birthday"
+
+## Environment variable with the sender's e-mail address.
+ENV_SENDER_ADDRESS: str = "GMAIL_ADDRESS"
+
+## Environment variable with the sender's app password (token).
+ENV_SENDER_TOKEN: str = "GMAIL_TOKEN"
diff --git a/tests/unit/test_constants.py b/tests/unit/test_constants.py
new file mode 100644
index 0000000..1ffecf5
--- /dev/null
+++ b/tests/unit/test_constants.py
@@ -0,0 +1,15 @@
+"""Smoke test: the package imports and the constants are consistent."""
+
+from birthday_wisher import constants
+
+
+def test_letter_file_pattern_has_number_placeholder() -> None:
+    assert "{number}" in constants.LETTER_FILE_PATTERN
+
+
+def test_csv_columns_start_with_name_and_email() -> None:
+    assert constants.CSV_COLUMNS[:2] == ("name", "email")
+
+
+def test_smtp_port_expects_starttls() -> None:
+    assert constants.SMTP_PORT == 587