SMTP Mail Issue

I have an automated system that generates PDF invoices and sends them to my customers on a regular basis. They are not receiving these e-mails.

I am using PHP on a Windows server to send these PDF as mime attachments using SMTP. We were using our ISP's SMTP, but now we are trying SMTP2Go corporate. We send the emails in batches of 250.

When I send these emails to myself, I get them on my company e-mail, my yahoo email, and my gmail. About 80% of these are getting delivered (out of about 220) daily. When we resend, they typically get delivered, about 99% of the time.

I am not getting any error messages our bounceback, I am not blacklisted, and we can send mail to these recipients. I used several tools to check my SPF and DNS settings and everything looks fine, I used the Port25 testing utilities and getting this:

SPF check:          neutral
DomainKeys check:   neutral
DKIM check:         pass
Sender-ID check:    neutral
SpamAssassin check: ham

I have many customers who have never had a receipt issue. I am also having the same issue whether I use my ISP (Comcast) or SMTP2Go's SMTP service - both require authentication and I tried multiple ports with both.

The server sending e-mail is an internal server that uses SMTP, it is not on the web.

E-mail is sending properly, it's just not being received.

The e-mails have a 3 sentance body, a subject, and a PDF attached and come from an actual sender e-mail.

I am at my wit's end; any suggestions would be greatly appreciated.

==========================================================
Details:
==========================================================

HELO hostname:  smtpcorp.com
Source IP:      174.143.60.32
mail-from:      yyyy@xxxx.com

----------------------------------------------------------
SPF check details:
----------------------------------------------------------
Result:         neutral (SPF-Result: None)
ID(s) verified: smtp.mail=yyyy@xxxx.com
DNS record(s):
    xxxx.com. SPF (no records)
    xxxx.com. 1200 IN TXT "i=240&m=domains-mx2-p12"

----------------------------------------------------------
DomainKeys check details:
----------------------------------------------------------
Result:         neutral (message not signed)
ID(s) verified: header.From=yyyy@xxxx.com
DNS record(s):

----------------------------------------------------------
DKIM check details:
----------------------------------------------------------
Result:         pass (signature verifies; identity doesn't match any
headers)
ID(s) verified: header.d=smtpcorp.com
CandidochrisAsked:
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DewFreakConnect With a Mentor Commented:
Add a SPF record - some systems will not accept without it so neutral is no good
Ensure your mail server IP is reversed

You are having to bend to ever filter out in the world that your customers use so at times this can be difficult.  You may need your clients to manually add you to allowed filters etc.

I would NOT use your ISP's mail server since it can become blacklisted at anytime, you have no control etc.  Not sure of this third party SMTP service.

Spend $15 a month and get your own mail server so you can tweak and control all aspects of it.

There are a lot of points of failure that can be happening here and you not being able to control the mail server or look at logs is an issue.
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NetExpert-WarszawaConnect With a Mentor Commented:
Are you able to look into your mail server logs? Probably not yourself, but could you ask some ISP's or SMTP2Go's support?  You need to know the response from a customers mail server.
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CandidochrisAuthor Commented:
NetExpert-Warszawa:Are you able to look into your mail server logs? Probably not yourself, but could you ask some ISP's or SMTP2Go's support?  You need to know the response from a customers mail server.
I have them looking at the logs, but I can't do it myself.

Add a SPF record - some systems will not accept without it so neutral is no good
Ensure your mail server IP is reversed

You are having to bend to ever filter out in the world that your customers use so at times this can be difficult.  You may need your clients to manually add you to allowed filters etc.

I have been actively working on this.

I would NOT use your ISP's mail server since it can become blacklisted at anytime, you have no control etc.  
I am not anymore.

Not sure of this third party SMTP service.
We started using them because they appear to have things covered pretty well.
http://corporate.smtp2go.com/howitworks/email-deliverability.php
 

Spend $15 a month and get your own mail server so you can tweak and control all aspects of it.
Any suggestions on where to do this for $15?

There are a lot of points of failure that can be happening here and you not being able to control the mail server or look at logs is an issue.

Agreed - thanks for the assistance.
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