How can I fix my smtp connection problem.

Are you running an Apple Xserve server with a mail server here (this forum) or are you using a Mac or iOS iPad or iPhone client and an ISP or other mail server hosted by somebody else? The former can be a little more complex than the latter.

If it's the latter case and it's a client that's having problems, then your SMTP server settings are incorrect. Mail clients have two entirely separate sets of server settings and credentials, one set for receiving and reading incoming mail via IMAP or POP, and a second and separate set of SMTP server settings and credentials for sending outgoing mail. Apple stores both sets of settings and credentials, but traditionally makes the latter set harder to get at.

Start here:

If you can’t send or receive email on your Mac - Apple Support

Mail settings you might need from your email provider - Apple Support

Mail outgoing server settings on Mac - Apple Support

Automatically manage connection settings in Mail on your Mac - Apple Support

To get to the outgoing SMTP server settings on iOS 11.4.1, launch the Settings app, select Accounts & Passwords, select the account that's causing problems, you'll get to the IMAP or POP settings and you'll need to select the account again, now scroll down and select the outgoing mail server and you'll get to a list of the existing SMTP server configuration settings, and you'll need to select the SMTP server you want to correct, and now (finally!) you can check and correct the settings here. See the list of mail settings you might need from your mail-hosting provider for what goes here. Now save and exit your settings, and try again.

Worst case? Remove and re-add the account entry, this after performing a complete backup using Time Machine or whatever other tool you're using to back up your user data and your mail data.

If you're running the Xserve server, then check the client settings above, and then you'll need to have a look at the server settings and network connectivity between the client and the server. Remote clients can need to connect to the submission port which is typically TCP port 586, and that'll need to be opened through any local network firewall. Don't assume remote clients can connect via TCP port 25, as that gets blocked in a number of remote networks. You'll need correct DNS settings and related details here too, though DNS is probably already working given that reading mail via POP or IMAP is working.