Cannot send email from RoundCube

[fantasymc]

Dear MRatWork Forum

I have a problem with email. i can get email from internet (gmail) but i can't send email to them (by reply or input address), this is notice
<chauict@gmail.com>:
User and password not set, continuing without authentication.
<chauict@gmail.com> 74.125.23.27 failed after I sent the message.
Remote host said: 550-5.7.1 [112.213.86.68] The IP address sending this message does not have a
550-5.7.1 PTR record setup. As a policy, Gmail does not accept messages from
550-5.7.1 IPs with missing PTR records. Please visit
550-5.7.1  https://support.google.com/mail/answer/81126#authentication for more
550 5.7.1 information. i8si22761404pap.189 - gsmtp

From   MAILER-DAEMON@mx8668.superdata.vn
To   khoatb@boluudiennctech.vn
Date   Today 10:28

I'm searched in internet and as my knowdlege this error cause i'm missing PTR record, but i'm check with http://www.intodns.com/ (for PTR recod) and http://mxtoolbox.com/ (for SPF record) => result is found.

So i don't know what my problem, could you tell me what should i do to fix this?
I'm sorry if this topic has same content with another.
Thank you for your help.

[MRatWork]

How many IPs in your server?.

[fantasymc]

Dear MRatWork,

i have only one IP.

[MRatWork]

Is still not work?.

[fantasymc]

Yes, still not work because i'm don't know how to fix so i still didn't touch.

P/S: Sometime your forum very slow and when reply it's show 500 error.

[MRatWork]

Yes, still not work because i'm don't know how to fix so i still didn't touch.

P/S: Sometime your forum very slow and when reply it's show 500 error.

I mean that you try send mail to gmail again?.

[fantasymc]

Yes, i was try many time (send email to gmail ) but still not working.

i can send email to this domain and send to my company email.

[MRatWork]

Check again in intodns.com and mxtoolbox.

[fantasymc]

Dear MratWork,

i did, everything ok, but still cannot send to Gmail.

you can see details as link below and also in attached images.

http://mxtoolbox.com/SuperTool.aspx?action=mx%3aboluudiennctech.vn&run=toolpage#
http://www.intodns.com/boluudiennctech.vn.

Could you tell me what i should do?

Thank you.

[MRatWork]

SOA serial still not match.

[fantasymc]

Dear Mrat,

Let's me tell you from beginning
Two week ago, my vps were be DDOS attack ( i guess so ), RAM Available <100MB (my VPS has 3.5 Gb Ram), MySQL stop working and website cannot access, it's show cannot connect to database, error (2).
i installed CSF firewall and now VPS working smoothly (maybe they don't want DDOS again)
Before installing CSF, i can send or receive email as normal. now i'm disabled CSF but i'm still send email to gmail/hotmail...
My mind i didn't change anything, exception installed CSF.
For SOA serial not match, could you tell me how to fix it?

Thank you.

[fantasymc]

Dear MRat,

Thank for your help, finally i'm contact with my VPS owner and they fix this.

The problem is : SMTP Reverse DNS Mismatch but i don't know what did they do