There are a few reasons your mailbox can reach huge sizes, here are a few and recommendations how to avoid them.
Sending files and photos via email attachments is very convenient, but e-mail is not suitable for sharing large files. There is no faster way to fill your e-mail account than to have your colleagues send you massive file attachments, or to save your files in your e-mail account!
For files larger than 3MB, we recommend using a file sharing service. There are many good ones that are freely available, such as Dropbox or Google Drive.
It is possible to have an email account configured to forward all messages to a different email account on a different domain. However, using this arrangement you will receive two times the amount of e-mails - those destined for this account, and those forwarded to it from the other mailbox.
The best we can recommend is to regularly delete e-mails you consider you no longer need and empty spam/junk and trash folders regularly.
Alternatively you can also use a mail client such as Outlook or Thunderbird to export e-mails to your local device and store them in case you need them at a later time. (You can do that with receipts of online payment, order confirmations, etc)
Updated 10 months ago