Instituto Superior Técnico

Serviços de Informática

Spam prevention systems

The email service includes an anti-spam global filter global to the Técnico email. The emails flagged by that filter with an high probability of being spam are automatically moved to a “SPAM” folder (or equivalent) in the user’s mailbox. Because false positives are possible we advise you to check this folder periodically or when some expected email seems to be missing.

Furthermore, to avoid abuse of the email service, there are limits to the number of emails (recipients) that can be sent in a set period of time. The limits are checked for intervals of 5 minutes, 1 hour and 1 day. The IT service will attempt to keep those limits as high as possible compatible with the need to avoid the blacklisting of the email servers. Every destination address included counts as one message towards this limit.

The restriction on number of emails sent are:
– 500 emails per day
– 450 emails per hour
– 200 emails in 5 min.
This was a necessary restriction because a large number of emails sent from a single sender is one of the factors weighted to evaluate the reputation of email servers. Such mass emailing occurs frequently when a a mail account is compromised and used by spammers, often before we can detect and close the account. Therefore the limits serve the purpose of restricting such damage. This kind of events have caused, in the past, our servers to be blocked by important email providers (such as Microsoft).

When a user triggers one of the limits, that will be explained in the error message received by the user. If the daily limit is reached the account will be prevented from sending email for 24h (from the first emails that were counted towards the limit). Before this daily limit is reached the 5 minutes and 1 hour limits will be triggered, so there should be sufficient warning to avoid it.

When sending a large number of emails is necessary the mailing lists service should be used.

The restriction on the size of each message sent is 25 MB. Due to encoding requirements the effective maximum size of an attachment is about 18Mb.