DKIM records have a maximum length of 255 characters, as defined in RFC 1035.
Records exceeding this limit will be rejected with the error "541 - Invalid attribute value; parse error in record .... one or more sections exceeding the limit of 255 bytes".
The correct way to enter such DKIM (or other TXT records exceeding 255 characters) is to split the value into multiple strings of 255 (or fewer) characters. Each string needs to be enclosed in double quotation marks (") using the following syntax:
Domain name TXT "String 1" "String 2" "String 3"….."String N"
Example DKIM DNS record:
_domainkey.example-dns.com IN TXT v=DKIM1; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEArJhbrJkGtc2DWV31AR/Rps43NoU9QwBCQMFnOpNk5JRCHe2iC+S1/6VhXqVchauoeLa18pHDGs8EEv5WXrzg8KRvWdu7lnpu5mYEpP3v4WmkEgF1CrKqx8TfOtY/wXToN3QEzGgRISwxKGOAZyECZzbFr0hSbA2+BtjC8LkYGJmwj77k13G1bclQf7EkgVvjQgbB8UIU631oWT8MwKAzGBWsu9iCjzSJpbGrcpQDoHZJgN+BNQknz36pRfcHFf/bJVRTrJX9rgGZBExcvBFNELqQodawun+EbZfT4u47Z474c7d46DOSMUe2KRfZOAkYovDVKs0Fxxf0BWy69IIiVwIDAQAB
Control Panel
To enter the above example DKIM record via the Control Panel, enter _domainkey as "Name", set type to "TXT" and copy the public key into the "TXT" field as below.
"v=DKIM1; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEArJhbrJkGtc2DWV31AR/Rps43NoU9QwBCQMFnOpNk5JRCHe2iC+S1/6VhXqVchauoeLa18pHDGs8EEv5WXrzg8KRvWdu7lnpu5mYEpP3v4WmkEgF1CrKqx8TfOtY/wXToN3QEzGgRISwxKGOAZyECZzbFr0hSbA2+BtjC8Lk" "YGJmwj77k13G1bclQf7EkgVvjQgbB8UIU631oWT8MwKAzGBWsu9iCjzSJpbGrcpQDoHZJgN+BNQknz36pRfcHFf/bJVRTrJX9rgGZBExcvBFNELqQodawun+EbZfT4u47Z474c7d46DOSMUe2KRfZOAkYovDVKs0Fxxf0BWy69IIiVwIDAQAB"
API
To add the DKIM / TXT record via API, use the following command syntax:
[COMMAND]
command=ModifyDnsZone
dnszone=example-dns.com
addrr0=_domainkey 28800 IN TXT "v=DKIM1; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEArJhbrJkGtc2DWV31AR/Rps43NoU9QwBCQMFnOpNk5JRCHe2iC+S1/6VhXqVchauoeLa18pHDGs8EEv5WXrzg8KRvWdu7lnpu5mYEpP3v4WmkEgF1CrKqx8TfOtY/wXToN3QEzGgRISwxKGOAZyECZzbFr0hSbA2+BtjC8Lk" "YGJmwj77k13G1bclQf7EkgVvjQgbB8UIU631oWT8MwKAzGBWsu9iCjzSJpbGrcpQDoHZJgN+BNQknz36pRfcHFf/bJVRTrJX9rgGZBExcvBFNELqQodawun+EbZfT4u47Z474c7d46DOSMUe2KRfZOAkYovDVKs0Fxxf0BWy69IIiVwIDAQAB"
As the public key has more than 255 characters, it was split into two strings enclosed in double quotation marks, separated with a blank space.