wp-includes/PHPMailer/PHPMailer.php:2985Assemble the message body.
string public function createBody() { $body = ''; //Create unique IDs and preset boundaries $this->setBoundaries(); $this->setWordWrap(); $bodyEncoding = $this->Encoding; $bodyCharSet = $this->CharSet; //Can we do a 7-bit downgrade? if ($this->UseSMTPUTF8) { $bodyEncoding = static::ENCODING_8BIT; } elseif (static::ENCODING_8BIT === $bodyEncoding && !$this->has8bitChars($this->Body)) { $bodyEncoding = static::ENCODING_7BIT; //All ISO 8859, Windows codepage and UTF-8 charsets are ascii compatible up to 7-bit $bodyCharSet = static::CHARSET_ASCII; } //If lines are too long, and we're not already using an encoding that will shorten them, //change to quoted-printable transfer encoding for the body part only if (static::ENCODING_BASE64 !== $this->Encoding && static::hasLineLongerThanMax($this->Body)) { $bodyEncoding = static::ENCODING_QUOTED_PRINTABLE; } $altBodyEncoding = $this->Encoding; $altBodyCharSet = $this->CharSet; //Can we do a 7-bit downgrade? if (static::ENCODING_8BIT === $altBodyEncoding && !$this->has8bitChars($this->AltBody)) { $altBodyEncoding = static::ENCODING_7BIT; //All ISO 8859, Windows codepage and UTF-8 charsets are ascii compatible up to 7-bit $altBodyCharSet = static::CHARSET_ASCII; } //If lines are too long, and we're not already using an encoding that will shorten them, //change to quoted-printable transfer encoding for the alt body part only if (static::ENCODING_BASE64 !== $altBodyEncoding && static::hasLineLongerThanMax($this->AltBody)) { $altBodyEncoding = static::ENCODING_QUOTED_PRINTABLE; } if ($this->sign_key_file) { $this->Encoding = $bodyEncoding; $body .= $this->getMailMIME() . static::$LE; } //Use this as a preamble in all multipart message types $mimepre = ''; switch ($this->message_type) { case 'inline': $body .= $mimepre; $body .= $this->getBoundary($this->boundary[1], $bodyCharSet, '', $bodyEncoding); $body .= $this->encodeString($this->Body, $bodyEncoding); $body .= static::$LE; $body .= $this->attachAll('inline', $this->boundary[1]); break; case 'attach': $body .= $mimepre; $body .= $this->getBoundary($this->boundary[1], $bodyCharSet, '', $bodyEncoding); $body .= $this->encodeString($this->Body, $bodyEncoding); $body .= static::$LE; $body .= $this->attachAll('attachment', $this->boundary[1]); break; case 'inline_attach': $body .= $mimepre; $body .= $this->textLine('--' . $this->boundary[1]); $body .= $this->headerLine('Content-Type', static::CONTENT_TYPE_MULTIPART_RELATED . ';'); $body .= $this->textLine(' boundary="' . $this->boundary[2] . '";'); $body .= $this->textLine(' type="' . static::CONTENT_TYPE_TEXT_HTML . '"'); $body .= static::$LE; $body .= $this->getBoundary($this->boundary[2], $bodyCharSet, '', $bodyEncoding); $body .= $this->encodeString($this->Body, $bodyEncoding); $body .= static::$LE; $body .= $this->attachAll('inline', $this->boundary[2]); $body .= static::$LE; $body .= $this->attachAll('attachment', $this->boundary[1]); break; case 'alt': $body .= $mimepre; $body .= $this->getBoundary( $this->boundary[1], $altBodyCharSet, static::CONTENT_TYPE_PLAINTEXT,Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.