When delivering email, an SMTP client, e.g., Mail User Agent (MUA), Mail Transfer Agent (MTA), uses the domain name system (DNS) to look up a Resource Record (RR) for the recipient's domain (the part of the email address to the right of the @); if there is a mail exchange Resource Record (MX record) then the returned MX record contains the name of the recipient's mailserver, otherwise the SMTP client uses an address record (A or AAAA). The MTA next connects to this server as an SMTP client. The local part of an email address has no significance for intermediate mail relay systems other than the final mailbox host. Email senders and intermediate relay systems must not assume it to be case-insensitive, since the final mailbox host may or may not treat it as such. A single mailbox may receive mail for multiple email addresses, if configured by the administrator. Conversely, a single email address may be the alias to a distribution list to many mailboxes. Email aliases, electronic mailing lists, sub-addressing, and catch-all addresses, the latter being mailboxes that receive messages regardless of the local part, are common patterns for achieving a variety of delivery goals.
Line speed, billed on the 95th percentile, refers to the speed in which data flows from the server or device, measured every 5 minutes for the month, and dropping the top 5% of measurements that are highest, and basing the usage for the month on the next-highest measurement. This is similar to a median measurement, which can be thought of as a 50th percentile measurement (with 50% of measurements above, and 50% of measurements below), whereas this sets the cutoff at 95th percentile, with 5% of measurements above the value, and 95% of measurements below the value. This is also known as Burstable billing. Line speed is measured in bits per second (or kilobits per second, megabits per second or gigabits per second).
The IETF's EAI Working group published RFC 6530 "Overview and Framework for Internationalized Email", which enabled non-ASCII characters to be used in both the local-parts and domain of an email address. RFC 6530 provides for email based on the UTF-8 encoding, which permits the full repertoire of Unicode. RFC 6531 provides a mechanism for SMTP servers to negotiate transmission of the SMTPUTF8 content.
Your next major concern will be compatibility. It's not a shock that most businesses run on Microsoft Windows and use some form of Microsoft Office. Being able to use common third-party clients such as Microsoft Outlook can often be a concern, and even today, compatibility with Microsoft Outlook isn't necessarily guaranteed. This is especially true when sending and receiving meeting invites. It only takes one garbled meeting invite to realize how frustrating this can be in the real world. Even if using Microsoft Outlook isn't a concern, portability is. If the service is entirely web-based, then is there a means for me to take my email offline and send email when I connect?
That has interesting implications: it means you can control what email gets downloaded when by having more than one account. I could separate askleo.com into two accounts, for example: one for the email addresses I want to pay attention to quickly, and the other for things that aren’t as critical. You could also segregate email based on which address it was sent to, which is what I do with my askleo.com email.
Some mail services support a tag included in the local-part, such that the address is an alias to a prefix of the local part. For example, the address [email protected] denotes the same delivery address as [email protected] RFC 5233, refers to this convention as sub-addressing, but it is also known as plus addressing, tagged addressing or mail extensions.
For those unlucky enough to choose an email host that doesn't have built-in spam detection, it can often be an ordeal to route email correctly through a third-party filtering service. Some businesses actually prefer engaging with a third-party spam filterer, mostly for compliance or customization reasons. But, for the majority of SMBs, this is headache they would be best off trying to avoid.