Spam mails are more than a mere nuisance for web hosts. When a server is spammed, valid mails get lost or delayed indefinitely in the pool of spam mails.
Equally panicky situation is outbound spamming from your server. Your mail server IP can easily get blacklisted, causing email delivery failures for your customers.
Spam mails are a nuisance! They are often silly and non-threatening mails coming to users’ inbox, and all the users may have to do is to delete it or mark it as spam.
But spam mails can quickly turn into terror when attackers spam server. Too much inbound spamming to your mail server can cause the mail queue to fill up and delay email delivery or even crash it.
These days, almost all business communications happen over email. If the mail server goes down for any reason, all mails in that server will bounce, causing significant business disruption.
For shared hosting providers, this is a major issue, as a single server hosts hundreds of businesses, and a service failure can affect all of them. During such a downtime, hosting providers are usually flooded with support requests to recover lost emails.