Bobcares.com provides outsourced hosting support to web hosts. As part of our services, we manage the tech support help desk of web hosts, and resolve issues posted by web hosting customers.

Many of our customers use Linux servers that run Exim as their mail server. Exim is usually present in web control panels such as Cpanel, DirectAdmin, Ajenti and more.

A common email error that we see in these servers is 435 Unable to authenticate at present. This is faced by email users when they try to send SMTP authenticated mail through the Exim server either through webmail (eg. Horde) or from an external mail service provider (Eg. Google’s Gmail).

What causes error 435 Unable to authenticate at present?

In short, this error happens when Exim is unable to check if the Username and Password given by the mail user is indeed correct.

It can happen due to:

• Failure of service that provides authentication (eg. Dovecot)
• Incorrect mailbox permissions
• User exceeding mail limits
• Incorrect SMTP Port settings at the email user’s client
• Corrupted auth libraries
• Disk quota issues

Let’s take a look at each of these issues one by one, and see how to fix it.

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Before we begin – a word of caution

Many server owners troubleshoot issues by trial and error. That is – Do X. Did it fix it? No? Then do Y.

It is not the best way to resolve issues, because:

• It takes a lot of time to go through all possibilities.
• A command executed without knowing the full impact of it could result in other services failing.
• In case something does go wrong, it’s hard to recover from it without knowing what changes the command made.
• Your server could have custom settings that can conflict with standard commands.

That is why we always recommend our customers to not make changes directly on the server, and instead escalate it to our 24/7 support team.

Our team uses an evidence based approach to solve issues. We look at logs, trace the sequence of events, and then find out exactly where the link is broken. It helps us solve issues faster, and prevent errors.

With that said, let’s now look at the top fixes for this error.

1. Quick fix – Restart POP/IMAP service

In some servers we’ve observed the POP/IMAP service failing to authenticate because of temporary data write issues.

So, a quick fix is to check if the database services are running OK, if the /tmp directory is clear, the disk health is OK, and then restart the POP/IMAP service.

In cPanel servers, this service is DoveCot.

2. Fix mail box permissions

When a login request is sent to POP/IMAP service, the server checks whether the mail directory is in fact accessible. If it is not, the IMAP server gives a fail signal to Exim, and this error will be shown.

So, if this account was recently migrated in, or was un-suspended, or any other recent changes were made, it’ll be a good idea to repair the mailbox permissions.

In cPanel servers, this can be done by:

1. Login to WHM
2. Locate the Email section in the left-side panel
3. Select Repair Mailbox permissions
4. Click Proceed.

Repair mailbox in WHM

3. Check mail limits

Many control panels allow mail limits to be set for each user. If the user has exceeded that limit for the time period (some times due to spamming via his account), no more mails can be sent.

So, look for the mail stats to see if this quota is exceeded.

4. Incorrect SMTP port

Many mail services (such as Google’s Gmail) does not allow outbound connections via port 25.

So, ask the customer if he has set the SMTP port as 25, and if so, to change it to port 587 or 465.

[ You don’t have to lose your sleep to keep your customers happy. Our friendly Hosting Support Specialists are online 24/7/365 to help your customers. ]

5. Corrupted authentication libraries

Exim and Dovecot depends on underlying system libraries to perform authentication functions.

In some servers which was recently upgraded, or otherwise a major change was performed, the libraries are likely to be corrupt.

To fix this, Exim should be re-installed.

In DirectAdmin servers, it can be done using CustomBuild, and in cPanel servers, this can be done with:

# /scripts/eximup –-force

IMPORTANT : This will remove all your customized settings, and could fail if you’ve protected your config file. Get expert assistance if you’re not sure.

6. Disk quota issues

In VPSs the disk space or inode limit could get exceeded causing temporary files and database writes to fail. This in turn results in a failed authentication.

So, if the server is prone to such issues, it’s a good idea to check that as well.

To conclude

“435 Unable to authenticate at present” is a common error in Exim servers. It happens when Exim is unable to authenticate an SMTP user. Today we’ve seen a few common reasons why this could happen. There could be more.

If you need assistance in fixing this error, we’d be glad to help. Click on this link to talk to our experts.