Nothing is endless, neither is the available space of your mailbox. Different hosting plans have different mailbox storage options. If you found out that your Mailbox is full, the first and easiest thing you can do, is make some room, by deleting old emails you don’t need any more.
A good place to start is by removing messages with large attachments. Remember that depending how you have setup your email client and/or device, besides the Inbox, there might be messages in folders like Sent, Drafts, Trash, or any other folder that you might have created. All of those messages occupy space.
You best bet is to Login to your Webmail and remove messages from there. Using Webmail you can be sure that the messages will be removed from your Mailbox.
When you’re done, don’t forget to empty the Trash too! 😉
What NOT to do
Your mailbox will be full much faster if you send or receive emails with large attachments. Any file larger than 1 Megabyte is considered large to be attached to an email message. You should avoid sending large files via email because both your and your recipients’ mailboxes will be full in a short period of time.
What to do
If you need to transfer large and non confidential files to other people you should really consider using other services like WeTransfer and Firefox Send, which were designed for that exact purpose. Mozilla Thunderbird has special integration with WeTransfer to simplify the process.