You may send any kind of attachment, as long as the resulting email is under 10MB in size (due to encoding issues this usually means an attachment of 6M). Attachments with names of the form 'filename.doc.exe' are commonly used for virus transmission, because under Windows they are shown as 'filename.doc' (hiding the fact that it is an executable program); these types of attachments are blocked for this reason.
FastMail.FM does not carry mailing lists used to distribute binaries (e. g. images, MP3s, etc.) for guest accounts. Messages to or from these lists will be rejected. Paid accounts do not have this limitation.
There are bandwidth limits dependent on your service level as detailed on the pricing details page .
There are limitations on sending and receiving emails designed to avoid mail abuse and keep FastMail.FM fast. Guests cannot send a message to more than 50 recipients at a time - simply send multiple messages with 50 recipients per message if you occasionally need to send to more people. Avoid sending more than 80 items in an hour (an item is one message to one recipient), and avoid having your email software check your email more than once every 5 minutes. If there are more than 200 logins to your account in an hour, you will be sent a warning mail. If the limit of 400 logins in an hour is crossed, your account will be locked up for an hour, and if it crosses 1000 logins an hour, your account will be locked up forever. For Guest users, these limits are kept at 130, 250 and 800 respectively. You will get a warning well before any action is taken, so keep an eye out on your mailbox if you're sending lots of email. Paid accounts have higher limits. If you approach these limits, you will get an email sent to your FastMail.FM address and to your backup address, which explains how to avoid the problem and what triggered it.
Bandwidth is one of the most costly parts of running an email service, and usually it's the top 0.1% of users who generate up to 50% of the total usage. The fact is most active users actually don't go over the 40MB provided to even just guests.
Eventually, every other service out there has to subsidize their bandwidth somehow. It might be through advertising, or a fixed yearly fee model through which some users pay for a lot more than they use, while others get a free ride subsidized by the low usage users. Who wants to subsidize greedy users who send/receive hundreds of large images or attachments each day, using your money and slowing down your email?
The reasons that many providers can get away without explicitly charging bandwidth is that:
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Many limit attachment sizes and recipient list numbers so that using lots of bandwidth is difficult
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Paid-only providers don't have to worry about bandwidth because only a small fraction of users would be loss-makers based on bandwidth
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Many providers have general comments about "reasonable usage" hidden deep in their terms of service, without explicitly saying how much bandwidth you can use
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Lots of providers that have failed to take these steps have gone out of business, citing problems with mail abuse
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Providers like Hotmail charge much higher fees for extra storage space than we do to indirectly offset the increased bandwidth you then generally use
Only a tiny percentage of people will have to think twice about bandwidth even on the cheaper "full" service level. But it's important for us because this group uses a large percentage of our total bandwidth.
Even users who use a lot of bandwidth may find FastMail.FM a superior service than other so called 'Free' providers. Read this email sent by one of our high bandwidth users to a number of his friends that shows why FastMail.FM charging for bandwidth even helps him.