Here I collect a few useful advise and tips that you may follow to take action or post to your website to help your customer keep the email communication with you.

Advise from Yahoo! mail:

My email is being blocked by Yahoo! Mail. What can I do?

How to properly prepare the email subject and contents.

We are experiencing delivery delays when sending to Yahoo! Mail. What can we do?

Any Concern about Gmail, please check this”
Does Gmail do a good job of filtering spam?

In General,

when your customer missed your email, what should you suggest them:

In Yahoo! Mail or Gmail, or any web-based email:

Locate the meaningful email from (spam) or (Trash)
move it by (Not Spam) or (move to ) Inbox

In Yahoo! Mail
Add the sender to Contact List

When  they are using an email client: (Outlook, Eudora,…)
Making a change to your email program.

Microsoft Outlook:

1. Go to Tools->Rules Wizard…
2. Click ‘New…’ (on the top right)
3. Choose ‘Check messages when they arrive’
4. Click ‘Next’.
5. Check ‘With specific words in the message header’.
6. Click on ’specific words’.
7. Type in: [SPAM: (this will catch-all messages with this in the subject)
8. Click ‘Ok’.
9. Click ‘Next’.
10. Check ‘Move it to the specified folder’.
11. Click on ’specified’.
12. Highlight an existing folder, or create a new one.
13. Click ‘Ok’.
14. Click ‘Next’.
15. Click ‘Next’. (Again, unless you want to add exceptions.)
16. Give the rule a name. (The default is what you typed for
specific words above.)
17. Check ‘Turn on this rule’. (You may or may not want to check
‘Run this rule on my Inbox now’.)
18. Click ‘Finish’.

Outlook Express:

1. Open Outlook Express as you would to check your email.
2. Then click on the tools menu. Click on Message Rules then click
mail. You will now see the message rules configuration.
3. Under ‘Select Conditions For Your Rule’, check ‘Where The
Subject Line Contains Specific Words’.
4. Under ‘Select Action For Your Rule’ check ‘Move It To The
Specified Folder’.
5. Under ‘Rule Description’ click contains specific words.
6. In the box that appears click in the textbox under ‘Type
Specific Words or Phrases’ and click ‘Add’.
7. Type in [SPAM: exactly as it is typed here and click ‘Add’.
8. Click ‘OK’.
9. Now click the ‘Underlined Specified’. In the box that appears
click on ‘Deleted Items’ and click ‘OK’.
10. Now click ‘OK’ in the ‘Message Rules’ box.
11. You should now see ‘Message Rules #1′ with a checkbox beside it.
Click ‘OK’ here.

Eudora:

1. Open Eudora as you would to check your email.
2. Next check for new messages and click on a message that you want
to filter.
3. If you have a [SPAM: email right-click once on that email and
then click on ‘Make Filter’. The ‘Make Filter’ dialog box opens. First
make sure the default checkbox for ‘Incoming’ is checked.
4. Next click the radio button below labeled ‘Subject.’ The subject
line from the selected email will already be in the box to the right.
5. Then in the ‘Action’ section click on the ‘Delete Message
(Transfer To Trash)’ radio button and the email will be deleted. (You
will still be able to review every message but you don’t have to. When
you empty your trash box the messages are deleted.) Finally click
‘Create Filter’ and the job is done!

Netscape:

Message filters allow you to manage and organize your messages.
You can create message filters that Netscape Mail uses to
automatically perform certain actions on incoming messages based on
criteria you specify. For example, you can create a message filter
that automatically files incoming messages in a particular folder.

If you are not already viewing the Message Filters dialog box,
begin from the Mail window:
1. Open the Edit menu and choose Message Filters. You see the
Message Filters dialog box.
2. If you have multiple mail accounts, choose the one to
which you want to apply the filter.
3. Click New. You use the Filter Rules dialog box to specify
the types of messages to act on, and the action you want the filter to
perform.
4. Type a name for the filter.
5. Select the matching option you want Mail to use: “all of
the following conditions” (criteria) you choose, or “at least one” of
the conditions.
6. Use the drop-down lists to choose the search criteria (for
example, “Subject,” “Sender,” “contains,” “doesn’t contain”) and then
type the text or phrase you want to match.
7. To restrict your filter to messages that only match a
certain priority, choose an option for priority from the drop-down
list.
8. Click More to add criteria and Fewer to remove them.
9. Use the drop-down list to choose the action you want the
filter to perform on the messages (for example, Move to Folder).
10. Choose a destination folder in which to store the
messages, or create a new folder.
11. Click OK to confirm your settings.
To manage your filters, begin from the Mail window:
1. Open the Edit menu and choose Message Filters. You see the
Message Filters dialog box.
2. If you have multiple mail accounts, choose the one to
which you want to apply the filter.
3. Choose from the following:
* To turn a filter on or off: Click the dot to the
right of the filter name to enable it (diamond), or click it again to
turn it off.
* To edit a filter: Select the filter name and click
Edit (or double-click the filter name).
* To delete a filter: Select the filter name and click Delete.
* To change the order in which filters are applied: In
the filter list, click a filter’s name, and click “Move Up” or “Move
Down” to move it.

Note: Filters are applied to each incoming message in the
order you choose, until a filter action results in the message being
deleted or moved to a different folder.
4. Click OK when you are done managing your filters. If you
created a new filter, it begins filtering incoming messages as soon as
you click OK.

Note: If you delete a folder that you’ve been using to store
filtered messages, the filter will no longer work. Incoming messages
that match the filter criteria will appear in your Inbox. If you
rename or move the folder, the filter will automatically update to use
the renamed or moved folder.

Tip: Filters don’t apply to existing messages. If you have
existing messages that you want to move to another folder, you can
search for those messages and then file them into the other folder.