The format of email addresses is [email protected] where the local part may be up to 64 octets long and the domain may have a maximum of 255 octets. The formal definitions are in RFC 5322 (sections 3.2.3 and 3.4.1) and RFC 5321—with a more readable form given in the informational RFC 3696 and the associated errata. Note that unlike the syntax of RFC 1034, and RFC 1035 there is no trailing period in the domain name.
It’s pretty much common knowledge that Gmail is awesome. It’s fast, connects seamlessly with the rest of your Google services such as Drive, has a cool app called Inbox, and is overall an extremely powerful email service. However, to use it with a custom domain, you need to purchase Google Apps for either $5 or $10/month, which for casual users is a bit unnecessary. On top of that, you don’t even get all of the features a personal account gets, e.g. Inbox.
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?