[fix] DKIM verification incorrectly succeeds with multiple TXT lines

## Describe the bug

**Node.js version:**

**OS version:**

**Description:** When setting up DKIM key I accidentally left a second line of the DKIM key (from a failed attempt at using 255+ character key) in my DNS records. The verification UI showed me that I had set everything up correctly... but all sent emails were of course being rejected by downstream mail servers.

## Actual behavior

DKIM verification succeeds if one of multiple available TXT record lines matches the required value.

## Expected behavior

DKIM fails if multiple TXT record lines are available while only a single record should be used.

## Code to reproduce

## Checklist

- [X] I have contacted support and they told me to raise an issue here.

The issue does not name a file, test, or entry point. Start by locating the DKIM verification path and reproduce the case with multiple TXT record lines; done means verification rejects multiple available lines instead of succeeding when one matches.

Written by the indexing model from the issue text.