Source: Google+ user Gmail 
Gmail's best features are tucked away in its nooks and crannies, meaning there are endless tricks you might not know about. Ahead, check out 20 essential pointers, and then add your own in the comments.
Want to get rid of those newsletters for good? There's an "unsubscribe" button next to the sender's address in promotional emails. It's a great one-click solution for decluttering your inbox! Another tip for the newsletters that don't get the "unsubscribe" button: type "unsubscribe" into the search box, and you'll probably find every deal, discount, and event email notification you've ever signed up for. You can also select all (but make sure to click "select all conversations that match this search" too) to banish historic newsletters from your inbox for good. Unroll.me  is a plug-in that can clean up your junk mail with just one click.
Is Someone Else Reading My Email?
If you've got a creeping suspicion that someone else is eyeing your inbox, check your "last account activity." In inbox view, scroll all the way down, and in the bottom right corner in tiny text, you'll see "Last account activity: X minutes ago." Click "Details," and Gmail will reveal exactly when each IP address accessed your account, plus what type of device they were using.
Default Reply All
Having to resend an email is the worst. If you're frequently part of mass emails, consider making "reply all" default. In Settings under the General tab, head to "Reply Behavior" and switch to "Reply All."
Select Multiple Messages
In Inbox view, holding down the Shift key while selecting messages will check multiple email threads at once.
Narrow Your Search
Typing in any of these operators will get you a more refined search.
- To find messages matching keyword A or keyword B, type "OR" in all caps, for example: from:amy OR from:david.
- Find specific types of files with "filename:" for example: filename:jpg.
- Search messages within a certain period with "after:" and "before:" for example: after:2014/04/30 before:2014/05/06.
- More here! 
Confront yourself with unread emails first thing every morning by setting up multiple inboxes. With this feature, you'll see your unread emails in one inbox on top, and everything else on the bottom. Head to Settings under the Inbox tab, then select "Unread First" under the Inbox Type drop-down menu.
Read Gmail Offline
The most underrated feature in all of Gmail? Gmail Offline.  It's a Chrome extension that allows you to read, respond to, search, and archive emails without an Internet connection.
Click on the gear icon (top right) > Settings, then click the Gmail Offline tab. You'll need to let the extension sync and download emails for a little while before you get offline. And don't worry — this extension doesn't take up any space on your hard drive.
Essential Keyboard Shortcuts
First things first: if you don't have keyboard shortcuts enabled, head to Settings, and under the General tab, turn keyboard shortcuts to "on." Once you have these enabled, just type one letter or a combination of letters to simplify everyday tasks. The ones you need to know are:
- j to go to older messages
- k to go to newer messages
- e to archive
- shift + # to trash
- / to type in the search bar
- a to reply all
- r to reply individually
Set Up Filters
Is a big-box retailer sending too many emails? If you don't want to see those emails show up in your inbox but would like to keep on receiving them (just in case): Select the message > More > Filter messages like these > Create filter with this search (you can add other keywords or filter out only messages with attachments, too) > Select Skip the Inbox (Archive It) and Apply the label (create a new label with the name of the company). Alternatively, you can set up emails from select friends and family to skip the inbox and be filtered into special labels.
Show Unread Messages in Tabs
Have no fewer than five tabs open at all times? You are a certified tab fiend. A Google Labs extension called "Unread message icon" shows you how many unread emails are in your inbox. It updates automatically so you don't have to keep opening your inbox — just glance quickly at the tab! Go to Settings > Labs tab > Enable "Unread message icon."
Send Emails From Different Addresses
Send mail from multiple accounts by heading to Settings and selecting Account and Import tab. In the "Send mail as" section, click on the Add another email address you own button, where you can add and verify all your external addresses. When you compose a message within Gmail, you'll then be able to select which email address you want to use from drop-down menu.
Schedule Email to Send Later
Have an email you're ready to write now but not ready to have the recipient read? Download the Boomerang Gmail plug-in  to schedule when an email should be sent. Choose a specific time to send the message like 8:45 a.m. when you know the recipient is just sitting down to email, or a more general time like "next Tuesday." Boomerang will also push read messages back up to the top of your inbox at whatever date and time you schedule.
Having email regret? Did you accidentally cc: someone you were supposed to bcc:? This feature could potentially save you from email blunder. Enable the "Undo Send" button by heading to Settings (the gear icon in the top right). Under the "Labs" tab, enable "Undo Send."
Easy Ways to Insert Photos
Simply paste, or control + V, images in the Compose window to insert a photo in your clipboard. If you use Google+ Auto Backup  (automatically uploads every photo taken on your mobile device to Google Drive), you can now insert those backed-up photos using the new Insert Photo  button in Gmail. You can also share entire albums.
Delete Your Biggest Emails
Approaching your 15GB  Gmail limit? That's not surprising since that number includes email, Google Drive, and Google+ Photos. Head here  if you want to see how much space you have left. Trash biggest messages immediately by searching "size:[x number]m" (including quotes is important), and the results will display all email bigger than [x number] of megabytes. Find Big Mail  is a web service that can also help.
Some email, like invites to weekend shindigs or nonpressing action items, you'd rather read later. A Google Chrome extension called Snooze Your Email for Gmail  puts a hold on those emails for hours or days, then bubbles them back up to the top with a desktop reminder. For other browsers, try this Google Apps Script .
Send and Archive
If inbox zero is what you're trying to achieve, this magic button will help you do it. Once you've responded to an email, that conversation will be moved away from the inbox into a digital filing cabinet. Archiving doesn't mean deleting mail. You can find archived messages in the All Mail tab on the left or with the search bar on top.
To enable Send and Archive, go to the gear icon (top right) > Settings. Under the General tab, find Send and Archive and select Show Send & Archive in reply.
Add Emails to Tasks
Responding to email takes a lot of time and energy. If it's not done immediately, it might never get done. Make sure important emails get attended to by adding them to Tasks. In the upper left, click the Gmail drop-down menu to open your Tasks window. To add an email to Tasks, select it from the inbox or open it, then click the More tab at the top. Select Add to Tasks.
In your Tasks window, you'll now see a list of emails that need responding to. Click on the task to go directly to the thread without having to search for it!
Drag Labels Onto Messages
This trick is an oldie but goodie. Drag and drop Gmail labels on top of threads in your inbox. It's so much easier than clicking on the Label tab and selecting the right category.
Make Your Inbox Comfier
If your inbox looks like a mess, give your eyes a break. In the gear icon on the top right, select the "Comfortable" display density. This will increase the space between emails, and add more white space in the right side navigation.