Can,t send emails to Gmail addresses due to SMP setting??

 lilyflds@gmail.com
   host gmail-smtp-in.l.google.com [142.251.111.26]
   SMTP error from remote mail server after pipelined end of data:
   550-5.7.26 This mail is unauthenticated, which poses a security risk to the
   550-5.7.26 sender and Gmail users, and has been blocked. The sender must
   550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,
   550-5.7.26 DKIM checks did not pass and SPF check for [southernphone.com.au]
   550-5.7.26 did not pass with ip: [54.160.229.207]. The sender should visit
   550-5.7.26  https://support.google.com/mail/answer/81126#authentication for
   550 5.7.26 instructions on setting up authentication. n7-20020a05622a040700b003e4e39b3567si577659qtx.593 - gsmtp

I can't send any emails via THunderbird to any Gmail address...is this a Thunderbird settings issue or do I need to sort it with my ISP? A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: lilyflds@gmail.com host gmail-smtp-in.l.google.com [142.251.111.26] SMTP error from remote mail server after pipelined end of data: 550-5.7.26 This mail is unauthenticated, which poses a security risk to the 550-5.7.26 sender and Gmail users, and has been blocked. The sender must 550-5.7.26 authenticate with at least one of SPF or DKIM. For this message, 550-5.7.26 DKIM checks did not pass and SPF check for [southernphone.com.au] 550-5.7.26 did not pass with ip: [54.160.229.207]. The sender should visit 550-5.7.26 https://support.google.com/mail/answer/81126#authentication for 550 5.7.26 instructions on setting up authentication. n7-20020a05622a040700b003e4e39b3567si577659qtx.593 - gsmtp