Kindly refer to the below steps on switching from SMTP port 25 to port 587 in your email client settings.
Microsoft Outlook 2007
1. Open Microsoft Outlook 2007. From the Tools menu select Account Settings.
2. Select your account under the E-mail tab, then click Change button.
3. The Internet E-mail Settings dialog box appears, showing the settings for your existing account. Click the More Settings button.
4. Go to Advanced tab, under Outgoing server (SMTP), change port 25 to 587.
5. Click the OK button.
Windows Mail
1. Start Windows Mail, click the Tools menu at the top of the window and then click Accounts.
2. Select your account under Mail, then click Properties button.
3. Go to Advanced tab, under Outgoing server (SMTP), change port 25 to 587.
4. Click the OK button.
Mozilla Thunderbird
1. Open Thunderbird Mail. From the Tools menu select Account Settings.
2. At the left pane, select the entry Outgoing Server (SMTP) and the settings will appear on the right.
3. Select your account, then click Edit.
4. Change port 25 to 587.
5. Click the OK button.
Apple Mail
1. Launch Apple Mail.
2. On the Mail Menu, click Preferences.
3. Select Account tabs and select your email account.
4. Click the "Server Settings" button under Outgoing Mail Server section.
5. Enter the "Outgoing Mail Server" field: mail.domain.com
6. Change the port from default 25 to 587