SPF check does not work on long DNS TXT records

Symptoms

• SPF does not work if SPF TXT record in DNS is too long.
• The following error can be seen in /var/log/maillog:

  CONFIG_TEXT: Mar 1 20:08:41 plesk01 spf[24253]: Starting the spf filter...
  Mar 1 20:08:42 plesk spf[24253]: Error code: (26) DNS lookup failure
  Mar 1 20:08:42 plesk spf[24253]: Failed to query MAIL-FROM: Temporary DNS failure for 'example.com'.
  Mar 1 20:08:42 plesk spf[24253]: SPF result: tempfail

Cause

It is caused by a bug in the product with ID #PPPM-8103 that will be fixed in future Plesk updates.

Resolution

As a workaround proceed to setup as follows:

1. Log into Plesk
2. Go to Tools and Settings > Server-Wide Mail Settings
3. Enable the option SPF (Only create Received-SPF headers, never block) 
4. Enable DKIM
5. Enable DMARC

Note: This way, DMARC will make decisions relying on SPF and DKIM status and the emails will not be dropped because of SPF problems but SPF status will still be included in DMARC policy check.