If your system is set to look first for an address record, known as an A record, it might not even attempt to find the MX record. Although it's technically incorrect to use an A record for mail, some sites do. You can eliminate many glitches by making sure your mail system uses MX records on all outgoing mail. Check the configuration options to see whether you can correct incoming mail that does not follow the rules. Accept A record lookups if you can.
Unreadable messages stem directly from the many formatting options available on mail readers.
Many people automatically put Hypertext Markup Language tags in their mail to add background colors, special fonts and graphics. These can get garbled in transit, particularly if they pass through a list server for mail list distribution. In some cases, a background color alone will render a message unreadable, even if most other HTML tags get through unharmed. To ensure legible messages, do not use HTML in any mail sent to a listserv computer, advised Greg Gunther, a technical support volunteer for a mail list I subscribe to. Select Plain Text.