Webmail vs Email Client (MUA) vs Mail Server (MTA/MDA) Explained

Webmail is an email client that runs in your browser and talks to the mail server directly on the provider side. An email client, formally a mail user agent (MUA), is an app such as Outlook, Apple Mail or Thunderbird installed on your device that talks to the server with IMAP and SMTP. The mail server is the software that actually transfers messages between domains (the MTA) and files them into mailboxes (the MDA). Understanding which part does what tells you where to look when something breaks.

Quick summary

• You write a message in an MUA, either webmail or an installed app.
• The MUA submits it to your provider MTA over SMTP port 587 (or the webmail backend does the same internally).
• Your MTA signs it with DKIM, looks up the recipient MX records (the settings that tell the internet where to deliver email for your domain), and transfers it over SMTP port 25 to the recipient MTA.
• The recipient MTA checks SPF, DKIM, DMARC and spam scores, then hands the message to the MDA.
• The MDA writes it into the right mailbox and folder, applying any filters.

The journey of one message

1. You write a message in an MUA, either webmail or an installed app.
2. The MUA submits it to your provider MTA over SMTP port 587 (or the webmail backend does the same internally).
3. Your MTA signs it with DKIM, looks up the recipient MX records (the settings that tell the internet where to deliver email for your domain), and transfers it over SMTP port 25 to the recipient MTA.
4. The recipient MTA checks SPF, DKIM, DMARC and spam scores, then hands the message to the MDA.
5. The MDA writes it into the right mailbox and folder, applying any filters.
6. The recipient MUA fetches it over IMAP port 993, or the webmail shows it from the same store.

The parts, defined

• MUA (mail user agent): what a person uses. Webmail, Outlook, Apple Mail, the Gmail app. It never talks to other domains; it only talks to its own server.
• MSA (mail submission agent): the port 587 front door of the server that accepts mail from authenticated MUAs. Usually part of the MTA.
• MTA (mail transfer agent): Postfix, Exim, Exchange transport, or a provider platform. It routes mail between servers using MX records and SMTP.
• MDA (mail delivery agent): Dovecot, Exchange mailbox store, or a provider store. It saves mail and serves it over IMAP and POP3.
• Webmail: an MUA hosted by the provider, so no ports or settings are needed on your side.

Example: the same account, three ways

Webmail        browser -> https://provider webmail -> mailbox store
Desktop app    Outlook  -> IMAP mail.oqumail.com:993 SSL/TLS (read)
                        -> SMTP mail.oqumail.com:587 STARTTLS (send)
Phone app      Apple Mail -> same IMAP and SMTP settings as the desktop

All three see the same folders because the MDA holds the only copy.

Webmail or an installed client?

• Webmail: nothing to configure, works on any computer, always current. Best for shared machines, quick checks and people who want zero setup.
• Installed client: offline access, unified inbox across several accounts, richer shortcuts and rules, native notifications. Best for people who live in email all day.
• Both at once is the normal arrangement: IMAP keeps them in sync, so choosing one does not lock you out of the other.

Where problems live

• Cannot log in from a mail app but webmail works: MUA settings, almost always the wrong port or encryption pairing.
• Mail sends but never arrives: MTA-level, check the delivery log for the remote server response.
• Mail arrives in webmail but not on the phone: MUA sync settings or a POP3 account deleting from the server.
• A folder exists on the laptop but not in webmail: the folder is local to that client, not on the MDA.

How OquMail fits

OquMail provides all three server roles and a webmail client. Sign in to webmail with no setup, or connect any MUA using mail.oqumail.com on 993 with SSL/TLS and mail.oqumail.com on 587 with STARTTLS. For software, the send API replaces the MUA and MSA entirely with an HTTPS request carrying a Bearer API key. Whatever the entry point, the same MTA signs with your DKIM key and the same delivery log records what the remote server said.

Common questions

Is Gmail webmail or a client?

The Gmail website is webmail for Google mailboxes. The Gmail mobile app is an installed client that can also connect to other providers over IMAP.

Do I need my own mail server?

No. A hosted provider runs the MTA and MDA. Running your own means managing reverse DNS, blocklists, TLS certificates and backups yourself.

Can two email clients use the same account?

Yes, with IMAP. Each client is just another window onto the same server-side mailbox.