I currently have a free StartSSL certificate for my mail server (postfix/dovecot). Of course, it works fine but Let's Encrypt certificates are easier and faster to work with. My Apache server also uses a Let's Encrypt certificate.
So, if I decide to use a Let's Encrypt certificate for my server, will other mail server reject my e-mails? How many acceptance has Let's Encrypt certificates in the mail world?
I know that every recent web browser accepts Let's Encrypt certificates. So, Let's Encrypt certificates are fine for web. Unless a user works with Windows XP and use a common browser from those times, the web will be properly displayed without browser diagnostic.
But how about a mail server? Does anybody have experience with Let's Encrypt certificates for mail? Specifically, my concern are not with big companies like Gmail, Yahoo or Hotmail, but with other private servers of other companies which perhaps doesn't accept my certificate.
NOTE I'm also worried about old mail clients like Outlook 2007 (some of my customer use them yet, and even Outlook Express), and smartphones (iPhones or Android).
ON-TOPIC EXTRA QUESTION Does Microsoft products delegates certificate management to the OS or other microsoft products? Because if I remember well, at least on Outlook 2003 and/or 2007, the certificate management was in charge of the IE browser; and if I remember well too, installing a user certificate on a browser like Firefox, effectively installed the certificate also on the OS itself (because I think it became system-wide available). So, if I'm right on that and even if a customer of mine has an old mail client, the certificate must be accepted automatically (guessing he has for sure a modern web browser accepting Let's Encrypt CAs), because root CAs from Chrome or Firefox are system-wide available.
In short: Should I take the risk of moving to Let's Encrypt for my mail server or it's better I wait at least one year more.