About this Add-on
You specify a maximum number of non-BCC (TO and CC) recipients and if this number is exceeded you will receive a warning when you send the email. You can continue or cancel. You can also move all recipients into BCC and send the email.
The option 'Strict Limit' ensures that you cannot send an email that exceeds the limit.
The companion addon, Expand mailing lists, will convert all mailing lists into their constituent recipients, removing duplicates.
TB 91 includes a built-in warning when too many public (TO or CC) recipient are entered.
The default value is 15 and can be set in Preferences, Config editor:
It does not check for duplicates (e.g. in lists) so may overestimate.
It does not count recipients in nested address lists (which are not recommended anyway).
TB 93 includes an optional additional check on sending, in case the first was ignored:
mail.compose.warn_public_recipients.aggressive (default is false)
That will warn again, but not prevent the mail being sent like 'Strict Limit' does.