Allow insecure authentication

After upgrading my iMac (late 2007 vintage) from OS10.9.5 to 10.11.6 I found that Mail will not send messages, with the error message:

"The server returned the error: Mail cannot send your password securely to the server. You can remove this restriction in the Accounts preferences by setting “Allow insecure authentication”, which could put your password at risk."

I first checked with my ISP/email provider, who advised me that due a difference between their system and Mail (which I won't pretend to understand), SSL must not be enabled. (Password is correct, and thus webmail access to my incoming mail is ok.) Advice was to contact Apple to see how to address SSL matter.

Apple Chat Support could not provide specific advice because the iMac in question is no longer supported but directed me to a https://support.apple.com/en-gb/guide/mail/cpmlprefacctadv/mac which seems to be tailored to OS10.14

After checking Mail>preferences>accounts I can see that SSL is already unchecked. (Password is correct, and thus webmail access to my incoming mail is ok.)

I later found the "Allow insecure authentication" option and unticked it, which has allowed me to receive email in Mail.

What this particular setting was before the upgrade I do not know. I've not changed the ISP/email details for several years. What are the implications of insecure authentication?

Mail version is 9.3.

Any insight appreciated please. Thank you.