Είναι να συναντηθώ με μια συνεργάτιδά μου στις έξι, αλλά εντωμεταξύ πρέπει να τελειώσω ένα πρόχειρο μήνυμα.

The pattern είναι να + verb means something like:

• I am supposed to...
• I am due to...
• It is arranged that I will...

So Είναι να συναντηθώ με μια συνεργάτιδά μου στις έξι means that the meeting is planned or scheduled for six.

This is not the basic way to say a future action. A simpler sentence could be:

• Θα συναντηθώ... = I will meet...

But είναι να... adds the idea of an arrangement, expectation, or plan already in place.

συναντηθώ is the aorist subjunctive form of συναντιέμαι / συναντώμαι, which is often used for meeting someone in the sense of getting together / meeting up.

Compare:

• συναντήσω = meet / encounter
• συναντηθώ = meet with / get together with

In everyday Greek, συναντηθώ με κάποιον is very common when two people are meeting each other by arrangement.

So here, συναντηθώ με μια συνεργάτιδά μου is the natural choice for meet with one of my colleagues.

Because the verb here is used with με to mean meet with someone:

• συναντιέμαι / συναντηθώ με κάποιον = meet with someone

So:

• συναντηθώ με μια συνεργάτιδά μου = meet with one of my colleagues

Even though English can just say meet someone, Greek very often uses με in this structure.

συνεργάτιδα means female colleague, female associate, or female coworker/partner, depending on context.

So:

• μια συνεργάτιδά μου = one of my female colleagues / a female associate of mine

The word is specifically feminine. If it were masculine, you would have:

• ένας συνεργάτης μου = one of my male colleagues

This happens because μου is an enclitic word, and Greek often adds an extra written accent when an enclitic follows a word whose normal stress is earlier in the word.

So:

• συνεργάτιδα
• but συνεργάτιδά μου

This extra accent helps show the correct pronunciation rhythm.

You will see the same thing in many similar phrases, for example:

• ο δάσκαλός μου = my teacher
• η θάλασσά μας = our sea

στις is the usual way to say at with clock times in Greek.

It comes from:

• σε + τις → στις

So:

• στις έξι = at six
• στις δύο = at two
• στις εννιά = at nine

This is just the normal time expression in Greek.

Yes, εντωμεταξύ is one word, and it means:

• meanwhile
• in the meantime

In this sentence:

• αλλά εντωμεταξύ πρέπει... = but in the meantime I have to...

It is a very common linking adverb used when one thing is scheduled for later, but something else needs to happen before then.

This is about aspect.

• τελειώσω is aorist subjunctive
• τελειώνω is present/imperfective

After πρέπει να, Greek often chooses aspect depending on how the action is viewed.

Here, τελειώσω presents the action as a single completed task:

• πρέπει να τελειώσω = I need to finish

If you said πρέπει να τελειώνω, it would sound more like a repeated, ongoing, or habitual sense, which does not fit as well here.

So the speaker means: there is one message, and they need to get it finished.

πρόχειρο can mean:

• rough
• quick
• preliminary
• draft-like
• not polished

So ένα πρόχειρο μήνυμα is a message that is not final or carefully refined yet. Depending on context, it could be:

• a rough message
• a quick draft message
• a preliminary message

It does not necessarily mean bad Greek or sloppy work. It often just means something prepared quickly before a more finished version.

Greek possessive pronouns like μου, σου, του, της, etc. usually come after the noun:

• η συνεργάτιδά μου = my colleague
• το βιβλίο μου = my book

This is the normal Greek order.

English says:

• my colleague

Greek says, more literally:

• the colleague of me

But in natural English translation, of course, we say my colleague.

Yes, it is natural, especially in spoken or informal written Greek.

The sentence sounds like someone explaining their schedule:

• Είναι να συναντηθώ... = there is a planned meeting
• αλλά εντωμεταξύ πρέπει... = but first something else must be done

A native speaker could also say similar things in other ways, for example:

• Θα συναντηθώ με μια συνεργάτιδά μου στις έξι, αλλά μέχρι τότε πρέπει να τελειώσω ένα πρόχειρο μήνυμα.
• Έχω να συναντηθώ με μια συνεργάτιδά μου στις έξι...

But the original sentence is perfectly understandable and idiomatic.