5.7.0 SMTP; 503 5.7.0 encryption too weak 0 less than 128

If you are receiving an error with wording similar to "encryption too weak 0 less than 128" then you are likely trying to send email to a mail server that requires encryption but sending from a mail server that does not support encryption.

A possible solution for Microsoft Exchange 2007 mail servers is :

Check the IMF threshold set on your exchange server. If it is set somewhere 6 6 then try to change to 7 6 and the restart SMTP and RE services. At the same time check the exclusions in your file level antivirus software (if any) installed on the exchange server.

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A possible solution for sendmail mail servers is removing lines such as the following from your sendmail.mc file :

TLS_Clt: ENCR:128
TLS_Srv: ENCR:128
TLS_Rcpt: ENCR:128