Recently most but not all messages I send to my wife's gmail address fail - the Message Delivery Failure notice contains the text below. I have done some research on "SPF" and "DKIM", so I know what the abbreviations stand for and something about their use. But I can't determine how either one might be implemented in T-bird to solve the problem. Both seem to be closely linked to the gmail ecosystem. I do not use gmail on T-Bird; all of my accounts are for/with my own '''.name''' domain name. This arrangement has worked very well for years, until this gmail issue started. Can anyone shed any light on it? Thanks in advance for any help or words of wisdom. Reason: There was an error while attempting to deliver your message with [Subject: "Text of LLC question"] to XXXXXXXXX@gmail.com. MTA p3plwbeout15-05.prod.phx3.secureserver.net received this response from the destination host IP - 172.253.122.27 - 550 , 550-5.7.26 This message does not pass authentication checks (SPF and DKIM both 550-5.7.26 do not pass). SPF check for [dunbar7.name] does not pass with ip: 550-5.7.26 [173.201.193.42].To best protect our users from spam, the message 550-5.7.26 has been blocked. Please visit 550-5.7.26 https://support.google.com/mail/answer/81126#authentication for more 550 5.7.26 information. 8-20020a370b08000000b0071f40a59fedsi3647616qkl.653 - gsmtp