EnterraHost
Mailbox storage is not fixed at the size you bought. Extra room is sold in blocks and can be added to any mailbox at any time, which makes a full mailbox an inconvenience rather than a migration.
How the extra storage works
Storage is bought in ten gigabyte blocks and added to the pool for the domain, then allocated to the mailboxes that need it. A mailbox that came as 10 GB can be given more without being rebuilt, and the mail already in it stays exactly where it is.
Because the pool is shared across the domain, you allocate it where it is needed rather than evenly. If one mailbox is doing the heavy lifting, it can take most of the extra and the others stay small, which is the point of pooling rather than buying everyone the same larger size.
The ceiling is 200 GB per mailbox. That is a very large mailbox by any measure, and reaching it means the mail is genuinely the archive rather than a working inbox.
How to tell you need more
The warning signs are less obvious than people expect, because a mailbox that is nearly full usually still works.
The clearest signal is mail that stops arriving with no local explanation. If somebody tells you a message bounced, and the senders are varied rather than one person, that is the pattern of a full mailbox rather than a filtering problem. Releasing a message from quarantine is the other cause worth ruling out, and it looks different.
Clients that repeatedly ask to rebuild a folder are the other one. A mailbox close to its limit can behave oddly in a desktop client long before it refuses mail outright.
The reliable answer is to check the storage figure in the mailbox settings rather than to diagnose from symptoms. It takes a moment and it settles the question.
Adding a block
Blocks are ordered from the mailbox's own settings, and they are billed per block rather than as a new mailbox. Nothing is interrupted while it is added, and no mail needs to be moved or re-downloaded.
Once the extra space exists, the mailbox uses it immediately. There is no rebuild step and nothing to reconfigure in your mail client, because the mailbox is the same mailbox with more room.
Not to be confused with the personal drive
Worth saying because the two are easy to mix up. Mailbox storage holds your mail. The personal drive is separate cloud storage inside the webmail for files you upload deliberately, and it has its own allowance on each tier.
Filling the personal drive does not affect mail, and adding mailbox storage does not enlarge the drive. If files are what is filling the space rather than messages, moving them to the drive is free where it exists, and cheaper than buying more mailbox to hold them.
If you are not sure which you need
Start with the smaller mailbox and add blocks as the storage fills. That is cheaper than buying the larger size in advance, and it costs nothing in flexibility because the upgrade is the same either way. Choosing between 10 GB and 50 GB covers what actually fills a mailbox, and the block price is on the business email page.