My Sendmail SMTP-relay experiencing problem as user sends email to over 70 users. the error message on sendmail log is (Sending error 421 4.7.0 Temporary System Problem), however the return fail message is (reason: 530 5.7.0 Must issue a STARTTLS command first).
About 60 users get the email out of 70, I don't know what is the problem. I browsed internet, some peole suggest to delay sending emails by 1000ms between each email. some other indcates the problem is with (using plain autentication). I basically don't know how to fix this:
Using following services:
SMTP-relay: installed on Linux system
user send email through SMTP-relay installed on the same server.
if you require other information please let me know.
your help is highly appreciated