Details: https://forum.cmsmadesimple.org/viewtopic.php?f=8&t=82641

"you are correct the limits are set in the siteprefs.tpl file in
admin/templates. The html limit is set to 50 and this governs the accepted
character length. The Db does not seem to constrain it as it is save as part of
an array.

Thanks for responding, I set the limit to 100 in the siteprefs file and all is
working as expected.

For anyone using sendgrid to relay smtp, the new API details require 62
characters to work, because the password field is hidden i did not notice the
capping of the string to 50, you will need to change the siteprefs file to allow
the api key to fit."