A while ago we posted an entry regarding YSmtp and apparently quite a few people visited this blog by searching "YSmtp service".
First of all, if you are a spammer, please leave immediately as the methods described here will not help you a single bit. We are sure you have better things to do hence please do not waste your time here. Second, if you are sending bulk emails to YSmtp, please read carefully what they told you (http://help.yahoo.com/l/us/yahoo/mail/postmaster/basics/postmaster-02.html) and contact them via email@example.com for assistance. Third, if you have not applied DKIM and DomainKeys on your clients' domain, go ahead and do them now. Forth, if your mail server is in US, sorry the tips below may not help.
So if you are not a spammer, not sending bulk emails to YSmtp, have applied DKIM and DomainKeys on the hosted domains, there are two workarounds that you may want to explore:
Option 1 - Pass the emails to your ISP
It is known that YSmtp whitelisted some government backed ISPs in various countries, and emails sent from these servers are guaranteed to be delivered. Now, using your ISP as a delivery proxy does come with a price - as most if not all of these SMTP servers require authentication before sending, your identity is revealed to the recipients and the email header will definitely appear funny.
For example, you are authenticated as firstname.lastname@example.org, but the FROM address would be your client's email address (such as email@example.com). Note that this option will fail if the SMTP server verifies the FROM address, i.e. authenticated email must be the same as FROM address.
With this method, the delivery path will look like this:
Client (Sender) -- Your SMTP Server -- ISP SMTP Server -- Recipient
Option 2 - Pass the emails to your VPS in US
We have not tested VPS in other parts of the world, however passing emails to a server located in US does help to improve the delivery rate dramatically. This is especially true for cases which the email is "big" (read the other entry for the definition of "big").
There isn't a need to get expensive VPS just for the purpose of this email proxy. Check the offers posted on LowEndBox and choose one that you are comfortable with. Any VPS with 128MB of RAM is more than sufficient in doing the job efficiently. Do make sure that the IP that the VPS provider allocated is not listed in any of the common blacklists (use RobTex to check).
The delivery path will be something like:
Client (Sender) -- Your SMTP Server -- Your VPS in US -- Recipient
And one last note, while email delivery is one aspect of the entire issue, emails landed in Spam folder is another, which by itself needs a separate post.