Add comprehensive tests for email "From" field validation

Implemented extensive test cases for the "From" field in email messages, covering various valid and invalid email formats according to RFC5322. Verified correct handling for each scenario to ensure robustness of email address validation.

msg_test.go

@@ -84,6 +84,41 @@ var (
 		{"1.0", MIME10, "1.0"},
 		{"1.1 (not a valid version at this time)", MIMEVersion("1.1"), "1.1"},
 	}
+	// Inspired by https://www.youtube.com/watch?v=xxX81WmXjPg&t=623s, yet, some assumptions in that video are
+	// incorrect for RFC5321/RFC5322 but rely on deprecated information from RFC822. The tests have been
+	// adjusted accordingly.
+	rfc5322Test = []struct {
+		value string
+		valid bool
+	}{
+		{"hi@domain.tld", true},
+		{"hi@", false},
+		{`hi+there@domain.tld`, true},
+		{"hi.there@domain.tld", true},
+		{"hi.@domain.tld", false},            // Point at the end of localpart is not allowed
+		{"hi..there@domain.tld", false},      // Double point is not allowed
+		{`!#$%&'(-/=?'@domain.tld`, false},   // Invalid characters
+		{"hi*there@domain.tld", true},        // * is allowed in localpart
+		{`#$%!^/&@domain.tld`, true},         // Allowed localpart characters
+		{"h(a)i@domain.tld", false},          // Not allowed to use parenthesis
+		{"(hi)there@domain.tld", false},      // The (hi) at the start is a comment which is allowed in RFC822 but not in RFC5322 anymore
+		{"hithere@domain.tld(tld)", true},    // The (tld) at the end is also a comment
+		{"hi@there@domain.tld", false},       // Can't have two @ signs
+		{`"hi@there"@domain.tld`, true},      // Quoted @-signs are allowed
+		{`"hi there"@domain.tld`, true},      // Quoted whitespaces are allowed
+		{`" "@domain.tld`, true},             // Still valid, since quoted
+		{`"<\"@\".!#%$@domain.tld"`, false},  // Quoting with illegal characters is not allowed
+		{`<\"@\\".!#%$@domain.tld`, false},   // Still a bunch of random illegal characters
+		{`hi"@"there@domain.tld`, false},     // Quotes must be dot-seperated
+		{`"<\"@\\".!.#%$@domain.tld`, false}, // Quote is escaped and dot-seperated which would be RFC822 compliant, but not RFC5322 compliant
+		{`hi\ there@domain.tld`, false},      // Spaces must be quoted
+		{`cow@[dead::beef]`, true},           // IPv6 is fine
+		{"hello@tld", true},                  // TLD is enough
+		{`你好@域名.顶级域名`, true},                 // We speak RFC6532
+		{"1@23456789", true},                 // Hypothetically valid, if somebody registers that TLD
+		{"1@[23456789]", false},              // While 23456789 is decimal for 1.101.236.21 it is not RFC5322 compliant
+		{"1@[1.101.236.21]", true},           // IPv4 is fine
+	}
 )
 
 //go:embed README.md
@@ -1113,6 +1148,71 @@ func TestMsg_EnvelopeFromFormat(t *testing.T) {
 	})
 }
 
+func TestMsg_From(t *testing.T) {
+	t.Run("From with valid address", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.From("toni.tester@example.com"); err != nil {
+			t.Fatalf("failed to set envelope from: %s", err)
+		}
+		addresses, ok := message.addrHeader[HeaderFrom]
+		if !ok {
+			t.Fatalf("failed to set envelope from, addrHeader field is not set")
+		}
+		if len(addresses) != 1 {
+			t.Errorf("failed to set envelope from, addrHeader value count is: %d, want: 1", len(addresses))
+		}
+		if addresses[0].Address != "toni.tester@example.com" {
+			t.Errorf("failed to set envelope from, addrHeader value is %s, want: %s", addresses[0].Address,
+				"toni.tester@example.com")
+		}
+		if addresses[0].String() != "<toni.tester@example.com>" {
+			t.Errorf("failed to set envelope from, addrHeader value is %s, want: %s", addresses[0].String(),
+				"<toni.tester@example.com>")
+		}
+		if addresses[0].Name != "" {
+			t.Errorf("failed to set envelope from, addrHeader name is %s, want: empty", addresses[0].Name)
+		}
+	})
+	t.Run("From with invalid address", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.From("invalid"); err == nil {
+			t.Fatalf("From should fail with invalid address")
+		}
+	})
+	t.Run("From with empty string should fail", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.From(""); err == nil {
+			t.Fatalf("From should fail with invalid address")
+		}
+	})
+	t.Run("From with different RFC5321 addresses", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		for _, tt := range rfc5322Test {
+			t.Run(tt.value, func(t *testing.T) {
+				err := message.From(tt.value)
+				if err != nil && tt.valid {
+					t.Errorf("From on address %s should succeed, but failed with: %s", tt.value, err)
+				}
+				if err == nil && !tt.valid {
+					t.Errorf("From on address %s should fail, but succeeded", tt.value)
+				}
+			})
+		}
+	})
+}
+
 /*
 // TestNewMsgWithMiddleware tests WithMiddleware