Some DNS editors can receive errors when DKIM keys or other TXT entries are longer than 255 characters.
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To resolve this issue, you can enter your DKIM key by breaking it up into pieces on a single TXT entry.
For example, if your DKIM key is:
"v=DKIM1; k=rsa; p=abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
You can enter it as:
"v=DKIM1; k=rsa; p=abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz" "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
You can divide them at arbitrary places. The double quote enclosed parts should be separated by a single space character, not a newline.
You can create a free OnDMARC account to test your DKIM configuration.