username

password

incoming server url & port & security type

SMTP outgoing server url& port & security type

POP3 is good for using email server as like a catch mailbox, like with option to keep or not keep email copy on email server

IMAP is good for using email server as like a sync center including folders

ref video: http://www.youtube.com/watch?v=GcvdhjLVYfY

mx record details can be found at Domain DNS setting > DNS zone files list > TXT record (includes SPF)

sender-policy-framework record is a text configure defining email activity on this email server, for others to verify whether this server's email is truely from this email server.

it helps avoid email sent from this email server been marked as junk/spam by others

all record should be ended with “-all” instead of “~all or ?all” to confirm verification tightness, like

"v=spf1 a mx ptr include:yourEmailServerURL.com -all "

ref: http://help.directadmin.com/item.php?id=207

SPF record syntax : http://www.openspf.org/SPF_Record_Syntax

SPF readup: https://www.digitalocean.com/community/tutorials/how-to-use-an-spf-record-to-prevent-spoofing-improve-e-mail-reliability

SPF video intro: http://www.youtube.com/watch?v=AFlXF6gPQGU

google email server example: https://support.google.com/a/answer/178723?hl=en

mail exchange record, tells others where the host's email server is.

https://dnschecker.org/all-dns-records-of-domain.php

https://docs.mailpoet.com/article/49-lists-of-hosts-and-their-sending-limits

solution: as it may be exceed server email sending limit, or maybe your email account info is stolen and been used by other for spamming. Change email password.